560 MR. R. LYDEKKER ON ZEUGT.ODONT [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 speciaHzed. 



The peculiar structure of the humerus induces me 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 ' 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 anv 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 posirion, 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 Zeuglodouts 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 diff'ereuces appear, 

 however, to me 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 Umb, can have "the closest possible relation" 

 with the Seals, passes my comprehension. 



I do not propose to discuss all the points raised in Prof. Thompson's 

 memoir, but it appears to me 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 me to be 

 merely another instance of the generalized characters of the 

 Zeuglodonts. In confirmation of this I may mention that Prof. 

 Cope ^ has recently figured the cervical vertebrae of an undoubted 



^ Stud. Mu3. Dundee, vol. i. art. 9 (1S90). 

 - Amer. ^at. 1890, p. 605, fig. 2. 



