6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 60 



had the teeth widely spaced throughout. This idea is strengthened 

 by the fact that the interspaces in the fragment preserved increase 

 in length toward the posterior end of the jaw, or maintain a con- 

 stant length, the length of each from in front backward being as 

 follows: 13, (hiatus), 20, 23, 23, 23 mm. In the type of Ischyro- 

 rhynchus the same intervals are about 10, 8, 4.5, 1,0,0 mm. The 

 mandible in the latter form is deeper than wide, while in Hespcro- 

 cetus, it is wider than deep, but this difference is rather specific than 

 generic. 



On account of the differences mentioned above, the assignment of 

 the Californian specimen to the genus Ischyrorhynchus does not 

 seem warranted. 



It is possible that Hesperocctus is related to the South American 

 genus Saurodclphis Burmeister, which belongs to the family Iniidse, 

 but as the mandible of the latter genus is not known this cannot be 

 determined at present. 



The genus Pontoplanodes Ameghino, which has already been 

 alluded to, presents some of the characteristics of Hesperocctus. 

 The mandibular teeth have conical, recurved crowns, with rugose 

 enamel, and in the anterior ones the root is more or less distinctly 

 divided into two or three branches at the extremity. The upper 

 borders of the jaw are sinuous in outline, and there are depressions 

 between the teeth. These depressions, however, are of small size 

 and are situated outside of the alveoli instead of in line with them, 

 as they are in Hesperocctus. The teeth are quite close together, 

 especially at the posterior end of the series. The mandible has a 

 distinct channel or sulcus, along the outer side of the symphyseal 

 portion, as in Platanista, etc. 



While Pontoplanodes presents some resemblances to Hesperocctus 

 they are not sufficient, in my opinion, to warrant a close association 

 of the two genera. 



Hcsperocetus may be provisionally assigned to the family Iniidse, 

 but it should be remarked that the teeth, though much larger, re- 

 semble those of Delphinodon which I have recently proposed to 

 transfer to the Delphinidse. No known genus- of that family, how- 

 ever, presents mandibular characteristics similar to those of Hcs- 

 perocetus. 



