192 A NEW FOSSIL PORPOISE FROM MARYLAND. 



In my opinion, Brandt's classification is the correct one. 



I am convinced also that Delphinodon belongs to the Delphinidse. We 

 have, therefore, two distinctive fossil representatives of this family, besides 

 Pithanodelphis Abel (1905), Delphinopsis Muller (1853), and the phocaenids 

 Protophoccena and P alceophoccena Abel. 



In the present form we have, in my opinion, one of the progenitors of the 

 recent Delphinidse. We learn from it that these dolphins were derived from 

 forms having teeth with crowns rugose and possessing three or more cusps, and 

 anterior and posterior ridges; the rostrum with a wide median interspace between 

 the premaxillse; and separate cervical vertebrae with complete vertebrarterial 

 foramina— the atlas with a complete foramen in the neural arch for the exit 



of the first spinal nerve. 



The rugosity of the enamel of the teeth and the anterior and posterior ridges 



are still retained in the recent genus Steno, while both that genus and Sotalia have 

 a rather wide interspace between the premaxillse. These characters are also 

 shared by the fossil genus Squalodon, but, in my opinion, it does not necessarily 

 follow that the delphinoids are derived from the squalodonts, although the 

 presumption is very strong in that direction. There are certain difficulties 

 which cannot be ignored. In the squalodont genera Prosqualodon and Dio- 

 chotichus, there are two transverse processes on each side of the atlas, while m 

 the delphinoid genera (the atypical Delphinapterus excepted) there is but one. 

 It is difficult to see exactly how the serrate crowns of the teeth of typical squalo- 

 donts could be transformed into the trituberculate crowns of Delphinodon. 

 The anterior and posterior longitudinal ridges do not represent a modification 

 of the squalodont cusps, because in Delphinodon the ridges and cusps, or tubercles, 

 are both present, the former running quite across the latter. Still, as certain 

 forms of Acrodelphis which may reasonably be considered as derived from the 

 squalodonts have teeth which present modifications of those of the latter group, 

 it is no doubt allowable to suppose that the teeth of Delphinodon may have had 

 a similar origin, or in other words, that it is derived from squalodontoid ancestors. 

 If such has been the fact, it is presumable that the delphinoid was not derived 

 directly from Squalodon, Prosqualodon, or their allies, but from other paralle 

 forms of a somewhat earlier age. The resemblance of the teeth of Delphinodon 

 to those of certain seals, such as Halichcerus, is very striking, and it is not o 

 wondered at that Leidy and other zoologists considered them at first as belonging 

 to pinnipeds. No one will suppose, however, that there is any direct connecti 

 between the latter and the delphinoid cetaceans. 



EXPLANATION OF PLATES XVII-XXVI 



PLATE XVII. 



Fig. 1. Skull, superior aspect. 



Fig. 2. Skull, inferior aspect. 



Fig. 3. Skull, lateral aspect. 



A little more than one-half natural size. 



