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Show 560 MR. R. LYDEKKER ON ZEUGLODONT [Nov. 1, shaft, and the circumstance that the radius and ulna articulated to the distal end by means of two slightly convex facets set obliquely to each other, the one being anterior and the other posterior. It is quite evident that there was but little, if any, movement between the humerus and the radius and ulna, and that the whole limb formed a paddle strictly comparable to that of modern Cetaceans, although probably somewhat less specialized. The peculiar structure of the humerus induces m e to take this opportunity of making a few observations as to the systematic position of Zeuglodon. It will be remembered that in a recent memoir Prof. D'Arcy Thompson 1 has endeavoured to show that these animals, in place of being Cetaceans, have *' the closest possible relation with the Pinnipedes." In that memoir the author has shown a wise discretion, from his point of view, in making no reference to the humerus. If that bone be compared with the humerus of a Seal it will be found that, beyond the fact that both have a strongly-developed deltoid crest, there is not the most remote resemblance between the two; the humerus of a Seal having condyles like those of any ordinary mammal. On the other hand, I submit, that between the Zeuglodont and Cetacean humerus there is a very pronounced general resemblance ; both having a very large head, directed outwardly in the natural position, a more or less flattened shaft, and the distal end terminating in fore-and-aft facets for the radius and ulna. It is true that in the Zeuglodonts the head is relatively smaller, the shaft longer and more flattened, and the distal facets more rounded than in modern Cetaceans ; while the latter have lost the distinct deltoid crest of the Zeuglodonts, and the two tuberosities have become confluent. These differences appear, however, to m e to be precisely those which we should expect to meet with in a generalized form ; and how it is possible to imagine that an animal with a humerus of this type, and a true paddle, in place of an ordinary fore limb, can have "the closest possible relation" with the Seals, passes m y comprehension. I do not propose to discuss all the points raised in Prof. Thompson's memoir, but it appears to m e that several of the characters he refers to as distinguishing the Zeuglodonts from typical Cetaceans and allying them to the Seals are likewise merely generalized features. For instance, the presence of a large spine and a small coracoid process in the Zeuglodont scapula appear clearly to come under this category, for I think few will deny that the enormous coracoid of the ordinary Cetacean is an acquired and not an inherited character, and the loss of the spine is assuredly so. Then, again, the cervical vertebrae, with their elongated centra, small lateral canals, and approximated upper and lower transverse processes, although undoubtedly very like those of a Seal, appear to m e to be merely another instance of the generalized characters of the Zeuglodonts. In confirmation of this I may mention that Prof. Cope2 has recently figured the cervical vertebrae of an undoubted 1 Stud. Mus. Dundee, vol. i. art. 9 (1890). 2 Amer. Nat. 1890, p. 605, fig. 2. |