mences; and, as the peduncles are broken across there, it is not possible to estimate their total length or the shape and size of the olfactory bulbs.

The coronal section formed by its anterior (broken) surface gives an isosceles triangle with a base measuring 8.5 mm. and sides of 10 mm. each. It expands as it passes backward, so that at its junction with the rest of the hemisphere its sides are each 19 mm. and its base 16 mm. in length.

FIG. 2.-Dorsal aspect of the artificial cranial cast of Zeuglodon. Two-ninths natural size. a, b, c, d, as in fig. 1. á, the dorsal rostrum, and b, an irregular boss on the cerebral hemisphere. (These are probably due to imperfections in the cranium.)

In the artificial cast (fig. 2) all that represents this extensive olfactory stalk is an irregular rostrum with two small bosslike projections, one above the other (a and á). The cerebral hemispheres in the natural cast have a broad base, from which

the sides extend upward toward the narrow dorsal surface with a gradual slope. In the artificial cast, however, the lateral parts of the hemispheres seem to be expanded into full rounded swellings.

Then, again, the antero-posterior diameter of the hemisphere is much shorter (being about 13 mm. less) than it is in the natural cast, although the breadth of the two specimens is approximately the same. It may be that the anterior parts of the skull, from which the artificial cast was made, are so damaged that little reliance can be placed upon the mold as an indication of the exact form of the brain. In fact, if this artificial cast even approximates to the form of the brain, it is quite certain that it did not belong to the same genus as the animal from which the natural cast was derived.

In other words, as we know that the artificial cast belonged to Zeuglodon, the probability is that the natural cast furnishes the first evidence of some hitherto undescribed genus of Archæoceti.

Behind the part b, which I have just described as the cerebrum, there is (in the natural cast) a large, irregular mass of a very peculiar shape, not exactly comparable to the condition occurring in any other brain known to me.

Immediately behind the hemispheres (b) there is a great transverse bar (c) measuring 125 mm. in the transverse direction—¿.e., extending on each side 15 mm. beyond the lateral margin of the cerebrum (6).

Each lateral extremity of this mass (c) is expanded to form a large buttress. In the natural cast these buttress-like masses are practically vertical, and of uniform thickness; whereas in the artificial cast they are obliquely-placed, and expanded ventrally. In the natural cast the mesial continuation of these thick lateral masses (each of which measures 30 mm. anteroposteriorly) becomes reduced to a bridge measuring only 5 or 6 mm. [the exact figure cannot be stated, because a piece of bone (fig. 1, e) partially covers this region].

In the deep concavity behind the narrow bridge of the area (in the natural cast) two rounded, irregular, walnut-like bosses

project, one on each side of the middle line (fig. 1, d). Each of these is 26 mm. in diameter, and is placed so obliquely that its surface looks almost directly backward. Shallow but clearly defined furrows separate these two bodies from each other and from the area c. In the artificial cast there is only a very faintly-marked indication of these bodies (fig. 2, d).

At a first glance it might seem that they represent the whole cerebellum, in which case c would be part of the cerebrum! But careful examination of the natural cast renders such an interpretation highly improbable, and comparison with the artificial cast seems to finally establish the belief that the whole of the region marked c forms part of the cerebellum.

It is extraordinarly difficult to accurately interpret this peculiar form of cerebellum. A comparison with other primitive types of cerebellum' points to the probability that the lateral buttresses of the mass c represent the floccular lobes, and that the walnut-like mass (d) represents the cerebellar lobule which I have called "area C" (op. cit., 'Catalogue,' p. 211). If it be objected that the lateral buttress-like mass is much too extensive to be entirely "floccular," attention may be called to the fact that in the large aquatic Sirenia, which have retained an exceedingly primitive type of brain, the floccular lobes are enormous in comparison with those of other mammals (op. cit., 'Catalogue,' p. 346).

It would perhaps be difficult to find elsewhere in the mammalia a greater contrast than that presented by the smooth, reptilian-like cerebral hemispheres of these casts and the highly complicated, ultra-mammalian neopallium of the recent whales, both Odontoceti and Mystacoceti. And yet, if we inquire into the nature of the factors which have molded the form and determined the size of the various parts of the brain in Eocene times and at the present, the contrast between the brain of Zeuglodon and the modern Cetacea loses much of its signific

1 Compare, for example (‘Catalogue of the Royal College of Surgeons,' 2nd edition, vol. 2, 1902), Armadillo (p. 211), Tapir (p. 311), Manatee (p. 346).

2 Vide 'Catalogue of the Royal College of Surgeons,' op. cit., pp. 348-359

ance, and becomes much less peculiar, even though it may not be wholly explained.

In most Eocene mammals the cerebral hemispheres were exceedingly diminutive in comparison with those of their modern descendants and successors. Moreover, the bulk of the primitive mammalian hemisphere was composed of those parts (hippocampus and lobus pyriformis), which are pre-eminently olfactory; in other words, the neopallium (i.e., that part of the pallium which is neither hippocampus nor pyriform lobe) is especially insignificant. It is a well-known fact that the sense of smell loses much of its importance in mammals of aquatic habits (e.g., Ornithorhynchus, the Sirenia, the Pinnipedia, and especially the Cetacea), and in these animals the olfactory parts of the brain dwindle to very small proportions. In the Odontoceti the olfactory bulb and its peduncle actually disappear. The Archæoceti, therefore, are subject to two factors, which will account in some measure for their small cerebrum. For, in addition to the smallness of the brain to which most Eocene mammals are subject, there is their aquatic mode of life. This causes a reduction in size of just those portions of the pallium which form the greater part of the Eocene hemispheres.

In the modern Cetacea the neopallium attains to the greatest absolute size which it ever reaches in any mammal. This fact cannot, however, be considered fatal to the belief in the close affinity of the Archæoceti and the Cetacea, because the extraordinary dissimilarity between the brains in the two sub-orders is such as we know to have been produced by the operation of well-recognised causes in the long lapse of time which separates the dawn of the Tertiary period from the present day. In all mammals which lead a life in the open" it has become a condition of their survival that the neopallium must increase in size in each successive generation: failing this, the creature must either adopt a "retired and safe mode of life" or become extinct. Numerous examples might be quoted in support of this hypothesis. But the case of the Sirenia shows us how little we really know of the factors which

determine the size of the brain. These creatures began the struggle for existence in Eocene times with relatively large brains, in spite of their aquatic mode of life; and they have been succeeded by generations of descendants whose latest progeny at the present day have a brain-equipment only slightly superior to their earlier Tertiary ancestors (vide Catalogue, op. cit. p. 344, et seq.). Even if we admit that the modern Manatees and Dugongs lead an eminently safe and retired life, which is in marked contrast to the venturesome and "open" life of the whales and porpoises, much still remains to be satisfactorily explained.

Perhaps the most striking feature of the brain of Zeuglodon is the extreme disproportion between the size of the enormous cerebellum and the diminutive cerebrum. In this respect the fossil brain presents a most marked contrast to that of all recent mammals, and especially to that of the Cetacea. This relatively great size of the cerebellum is not peculiar to the Archæoceti, but is common to other extinct mammals of large size. In my memoir on the brain in the Edentata1 the difficulty presented itself of adequately explaining a similar phenomenon in Glyptodon; and it must be born in mind, in even attempting to do this, (1) that the obtrusive greatness of the cerebellum presents itself only in large mammals and not in lowlier vertebrates, and (2) that the size of the cerebellum is not proportionate to that of the cerebrum. In the case of Glyptodon I four years ago attempted to explain these facts in this manner.

The development of the neopallium in mammals opens up the possibility of the performance of many more complex muscular acts than are possible in the Amphibia or Reptilia; these acts require a co-ordinating mechanism, the size of which will be largely determined by the bulk of the muscular masses, the actions of which are to be harmonised, and the extent of the sensory surfaces which send into the cerebellum streams of controlling impulses. A large cerebellum is being demanded by a

1 "The Brain in the Edentata," Linnean Society's Trans.,' 2nd series, Zoology, vol. 7, part 7, 1899, p. 381.