604 The American Naturalist. [July, 
II. Teeth with cylindric roots. 
a, Caudal vertebr& plano-convex. 
No caudal diapophyses ; Cetophis Cope. 
aa, Caudal vertebrze plane. 
f, Lumbar diapophyses spiniform. 
Lumbar and caudal vertebra slender ; Zarhachis Cope. 
Lumbar and caudal vertebr& short; Ixacanthus Cope. 
* BB, Lumbar diapophyses wide, flat. 
Muzzle elongate, slender ; cervical verte- 
brz long; Priscodelphinus Leidy. 
Muzzle slender; cervical vertebrz shorter ; Pontoporia Gr. 
III. Teeth with longitudinally flattened roots. 
Teeth in entire length of maxillary bone; sym- 
physis connate; | Stenodelphis Gerv. 
Teeth on all the jaws ; symphysis not connate ; 
an erect osseous crest on posterior part of 
maxillary ; Platanista Cuv. 
Teeth at the base of the maxillary only ; muzzle 
produced into a sub-cylindrical beak ; Rhabdosteus Cope. 
IV. No teeth ; an alveolar groove. 
Muzzle depressed, elongate ; Agabelus Cope. 
The recent species belong to the genera Inia, Pontoporia and 
Platanista. The two first are found in the rivers of S. America, 
and the Platanista gangetica in the rivers of India. Their posterior 
ribs are one-headed. The genera with spiniform diapophyses of 
the posterior vertebrz are only known so far from N. America. 
The Zzacanthus celospondylus Cope was a short robust species 
about the size of a white whale. Another line of modification is 
seen in the attenuation of the vertebral column. The most 
remarkable elongation of the vertebrze is found in Zarhachis, a 
character which is only paralleled in Zeuglodon. Of the other 
genera, Stenodelphis, with its single species S. canaliculatus (Del- 
phinus, von Meyer), has been so far found in the middle Miocene 
of Central Europe. Priscodelphinus occurs in the Miocene of 
North America and Europe. The P. grandaevus Leidy (Figs. 2 
and 3), of the Miocene of New Jersey has a slender muzzle, with 
a full series of curved cylindric teeth; a neck like that of a seal 
