EASTMAN: TYPES OF FOSSIL CETACEANS. 87 



We may now return to the principal matter at issue, namely, a comparison 

 between Lophocetus and certain fossil genera which are regarded as standing in 

 close relations with Inia, and are commonly assigned to the same subfamily. 

 Now, the greater number of fossil Platanistids, or Iniidae in Gill's sense of the 

 term, are remarkable for having the rostrum greatly elongated. In recognition 

 of this fact, Abel divides his so-called Acrodelphidae into two sections, the first 

 three subfamilies listed above being embraced in a section of ' Longirostres,' and 

 the fourth, containing only Delphinapterus and Monodon, constituting the ' Brevi- 

 rostres.' At first sight these longirostrate Platanistids would seem to present a 

 marked difference from Lophocetus, for, as noted by Harlan, it does not appear 

 that the rostrum in this form was greatly produced, and probably not more than 

 a few inches are missing from it in its present condition. The solidity of the parts 

 composing the muzzle, and general resemblance of the latter to that in breviros- 

 trate Delphinoids, are in harmony with Harlan's conclusion, and so also are the 

 facts of geographical distribution. Longirostrate Platanistids are especially char- 

 acteristic of European Tertiary deposits, whereas on this side of the Atlantic forms 

 like Champsodelphis, 1 Schizodelphis, Eurhinodelphis, etc., are conspicuously 

 absent, being replaced, apparently, by brevirostrate genera. Probably the expla- 

 nation of this fact is to be found in differences of physical conditions, such as are 

 to be inferred from the different constitution of the faunas as a whole, and from 

 the different nature of the sediments composing the deposits. 



The Miocene deposits of the Middle Atlantic Slope in this country are of char- 

 acteristically marine type, as indicated by both structural and fossiliferous evi- 

 dence. On the other hand the corresponding Old World formations from which 

 Delphinoid remains have been obtained are on the whole less clearly of marine 

 origin, and the very circumstance that most of these Delphinoids are longiros- 

 trate has been interpreted in the light of adaptation to estuarine or even fluviatile 

 conditions. For as shown by Dollo 2 and various other writers, it is precisely 

 this modification that is oftenest met with in widely diverse orders of vertebrates 

 where forms have become adapted to a littoral or fluviatile existence, as for 

 instance, Lepidosteus among fishes, and Champsosaurus, Phytosaurs, and modern 

 and extinct gavials among reptiles. Dr. J. H. McGregor, 3 in his memoir on the 

 Phytosauria, calls attention to the striking resemblance of the rostrum to the 

 snout of Lepidosteus, and quotes Fraas's observation that its decurved tip " per- 

 haps demonstrates a habit of rooting in mud for food, and catching fishes." 

 Cope, 4 also, noted a somewhat analogous formation of the rostral portion of the 

 jaw in Anoplonassa, and offered a similar explanation. And more recently, the 

 same conclusion has been put forward by Abel 5 in following language : — 



1 The reference to this genus of certain detached teeth and vertebrae from the 

 Maryland Miocene must be regarded as provisional only. 



2 Nouvelle note sur le Charapsosaure, Bull. Soc. Beige Geol., 1891, 5, p. 153. 



3 Memoirs Amer. Mus. Nat, Hist., 1906, 9, p. 38. 



4 Proc. Amer. Philos. Soc., 1869, 11, p. 189. 



5 Mem. Mus. Roy. d'Hist. Nat. Belg., 1905, 3, p. 154. 



