186 PROCEEDINGS OF THE ACADEMY OF 



cutting. Roofs of premolars compressed." (P. 152): "Two teeth having 

 crowns similarly though rather more sj-mmetricallj' formed, I suspect to have 

 occupied that position," i. e. " between molars and premolars." 



The premolar first mentioned is about -8 in length, the crown slightly striate. 

 The anterior molar has two roots, which are united some distance below the 

 crown. The crown exhibits no denticles, and is not more elevated than the 

 antero-posterior diameter of its base. This tooth, as well as the other, is of 

 small size, and indicates the smallest species of the family. 



The Pontogeneus priscus Leidy, which I referred, in accordance with a 

 printed suggestion of Leidy's, to the Zcuglodon pygmcEUs Miiller, on examination 

 proves to be a Delphinoid. The species of Miiller is not only generically dis- 

 tinct from Basilosaurus, but from Doryodon also, to which I referred it,* if it 

 be regarded as established on the cranium figured by Gibbes in *he Journal of 

 the Academy. 



DELPHINIDjE. 



Among the vertebras of the species of this family collected by Jas. H. 

 Thomas in the Miocene marls of Charles county, Maryland, may be recognized 

 those of five genera, as follows : 



The caudal vertebrae broader than long; lumbars, sacrals and caudals nearly 

 similar; diapophyses of Inmbars and caudals flat dilated, the latter with 



vertical foramen Delphinus. 



The caudals longer than broad, slender ; diapophyses broad, of caudals per- 

 forate Delphinapterus. 



The caudal vertebrfe longer than broad, lumbars and caudals with flat diapo- 

 physes which are not perforate.. Zarhachis. 



The lumbars, dorsals, and caudals elongate, narrower than long; the diapo- 

 physes of some of the lumbars and of the caudals narrow and spinous, and 



not perforate; epiphysial face plane Priscodelphinus. 



The lumbars and dcrsals shorter, the diapophyses of both posterior lumbars and 

 caudals narrow and subcylindric, not perforate ; epiphysial face grasping 

 the epiphj'sis by four processes, one opposite each neur- and one opposite 



each diapophysis Ixacanthus. 



We find a serial relation among the Dolphins of this period, and exemplified 

 in the characters of the above genera. In Tretosphys the diapophyses are all 

 flat as in Dolphins, generally many of the caudals perforate, with a vertical 

 foramen at the base as in them. This is succeeded by a genus in which the 

 blood vessel which in the former passes through this foramen runs round the 

 front of the base of the diapophysis. In the next form some of those of the 

 caudal vertebrte are narrower, and the posterior subcylindric and spine-like; 

 in the last genus of the series the diapophyses of all the caudals and many of 

 the lumbars have the same spinous form. There is also a relation of a similar 

 kind in the forms of the beak of Miocene Dolphins. All are elongate, some 

 very narrow and prolonged, and some acylindric beak only toothed at the base, 

 (Rhabdosteus Cope). It is an interesting object of inquiry to determine 

 whether the relation of structure of the processes in any way coincides with 

 that seen in the muzzle. 



A point to be noticed in our Miocene Dolphins, as compared with the Inia, Be- 

 luga, Delphinus and Phocaena of the present period, is the universally increased 

 length of the vertebras of the posterior part of the vertebral column. Those 

 species named here Delphinapterus resemble in their dorsal vertebrae the Belu- 

 gas, but the caudals of some, instead of being shortened, as in the latter, do 

 not diminish in length. This points to a more slender form, and with the 

 narrowed diapophyses and increasing thickness of the epiphyses constitutes an 

 approach to the Basilosaurus type. 



* Proc. Acad. 18G7, 155. 



[July, 



