NO. II FOSSIL TOOTHED CETACEAN— TR1 I 5 



diameter of the mandible a little greater than the largest transverse 

 diameter. The upper part terminates in a rounded longitudinal emi- 

 nence (quilla). Roots of the teeth less compressed than in Saurocetes 

 [=zPoutoplaiwdcs], frequently imperfeet at the termination below, 

 with the two points, anterior and posterior, less distinct. Crowns 

 of the teeth conical and low, with the apex turned a little backward 

 and the enamel strongly rugose. Teeth implanted in an alternating 

 order, each of those of the right side opposite the space which inter- 

 venes between two of the left side, and vice versa with each of the 

 teeth of the left side in relation to those of the right side. Average 

 diameter of each of the posterior molars at the level of the alveolar 

 border: antero-posterior, 13 millimeters, transverse, 8. Diameter, 

 antero-posterior, of the crown of a detached tooth, 9 millimeters ; 

 transverse, 9 millimeters. Height of crown, 11 millimeters. Trans- 

 verse diameter of the beak at the anterior end of the part figured. 31 

 millimeters ; vertical diameter, 38 millimeters." ' 



In Hesperocetus the teeth of the two sides are opposite, but this 

 might be, and probably was, so in the anterior part of the mandible 

 of Ischyrorhynchus. Displacements of the teeth similar to that 

 occurring in the latter genus are common in various recent genera 

 of Delphinidae. In Ischyrorhynchus the teeth are close together, or 

 even in contact, on each side at the posterior end of the series 

 figured by Ameghino, but the anterior ones are separated by inter- 

 spaces which increase in length forward. It is not improbable, 

 therefore, that near the anterior end of the complete jaw they were 

 quite as widely spaced as in Hesperocetus. 



There is also some indication of depressions in the interspaces 

 between the teeth in the fragment figured by Ameghino, but they are 

 quite indistinct, and are somewhat external to the alveoli, rather 

 than in line with them, as in Hesperocetus. 



Although the dimensions of the teeth in the two forms are nearh 

 identical, the proportions of the jaw do not agree. The fragment 

 on which Ischyrorhynchus is based, if it were from the same species 

 as that on which Hesperocetus is based, should be from nearer the 

 posterior end of the symphysis, on account of the close approxima- 

 tion of the teeth in the former, [t should, therefore, be broader. 

 The type-fragment of Ischyrorhynchus is, however, narrower at the 

 posterior end than the Californian specimen. From this circum- 

 stance, it seems reasonable to suppose that the latter, when complete. 



'Revis-.a Argent. Hist. Nat, Buenos Aires, vol. 1. 1891, pp. 163 [65 



