10 PROCEEDINGS OF THE ACADEMY OP 



The animal to which it belonged was not less attenuated in the posterior part 

 of the vertebral column than the great Basilosaurus. 



Zarhachis velox Cope. 



This species is likewise only represented by a single vertebra, which is from 

 the lumbar series anterior to the position of that of Z. tysonii just de- 

 scribed. It has pertained to an adult animal of half the size of the preceding, 

 and one which carried the narrowed subs])iiious diapophyses forward, though 

 perhaps not so markedly as the Friscodelphinus s p i n o s u s, m. 



The inferior outline is straight, and is the edge of a very strong thin keel, whose 

 greater median prominence is due to the strong concavity of the inferior sur- 

 faces. The same concavity with that of the upper surface causes the existence 

 of a strong longitudinal lateral keel, from the middle of which springs the 

 diapophysis. The basis of the neural arch is thin and does not extend over 

 more than -6 the length of the centrum. 



The articular faces are discoid, and if one diameter exceed another it is the 

 vertical ; they have a somewhat expanded appearance from the concavity of 

 the sides. Surfaces smooth. 



Lines. 



Length centrum 33 



Transverse diameter of extremity 17 



Vertical " " 17 



Internal width of neural canal 31 



Length of basis of diapophysis 8 



This species was taken from the rniocene marl from the pits of Reuben 

 Ayers, near Shiloh, Cumberland Co., N. Jers.-y. 



It indicates an even more slender and snake-like cetacean than the preced- 

 ing, of much smaller size. 



ESCHRICHTIUS Gray. 



There is in the Thomas collection a portion of the cranium of a small 

 Balspnoid, which from its resemblance to those of the existing tinner whales, 

 its small size, locality, and black color, I attribute provisionally to the Esch- 

 richtius p u s i 1 1 u s. It serves to confirm the affinities expressed in the name 

 established on the ramus of the mandible. The alisphenoids present a deep, 

 smooth posterior excavation, as in Sibbaldius, while the infero-lateral processes 

 of the basioccipital are stronger than in that genus. The conchs of the per- 

 otic bones are preserved ; they are characterized by the possession of a hooked 

 process turned outward, on the outer and more elevated margin. 



An examination of additional material of these extinct Bal^nidie, has 

 enabled me to trace the affinities of species of which little has been hitherto 

 known. Thus the Balsena prisca Leidy appears nearly affined to species re- 

 ferred by me to Eschrichtius through intermediate forms. Vertebrae very sim- 

 ilar to those referred to the Mcgaptera expansa Cope are accompanied by man- 

 dibular rami of the same general type, and would be better referred to the 

 same genus, in the absence of evidence to the contrary. It appears that there 

 are six sfiecies of the genus, whose characters offer nothing as yet to separate 

 them from the scarcely extinct type E. r o b u s t u s Lillj. Five of these can 

 be characterized from the forms of their mandibular rami, and are therefore 

 compared in the following table. The other species, E. leptocentrus, m., 

 is indicated by vertebra? alone. 



Much compressed, outer face little convex; superior margin a narrow ridge 

 without any truncation, with a series of foramina on each side, the inner ex- 

 tending for a very short distance only ; no marginal groove ; inferior edge 

 narrow. Very large E. cephalu.s. 



Upper edge broad, with outer series of foramina, and meeting inner-edge at 

 a right angle, which is the highest line, and with inner series of foramina just 

 below it ; most convex externally. Large fi--- E. piuscus. 



[March, 



