42 SYSTEMATIC PALEONTOLOGY 



produced forwards, so that the length of the cranium from the posterior 

 border of the frontal bone is one and one-half times as long as the 

 depth of the cranium at the same point. 



" The tympanic bulla has the general form characteristic of species 

 of this genus, but presents specific characters of its own. The part 

 anterior to the posterior boundary of the external process is half as 

 long again as the length posterior to it. The two measurements are 

 equal in the C. hupscliii, according to A^an Beneden. The two ridges 

 of the internal border unite 19 mm. posterior to the anterior extremity, 

 forming a single acute angle. This character is not described by authors 

 as occurring in any other species of this genus. The anterior extremity 

 is squarely truncate, and is semicircular in outline, as the superior side is 

 fiat and the inferior convex. In C. hrialmontii, according to Van Bene- 

 den, the bulla is not truncate in front nor is there a single acute edge on 

 the inner side in front; the portions of the bulla anterior and posterior 

 to the internal process are of equal transverse Avidth; in the C.megalo- 

 physum the anterior portion is considerably narrower than the posterior 

 portion. In Mesocetus agramii, according to Van Beneden, there is a 

 single acute internal ridge on the bulla, but it is much longer than in 

 the C etotherkim megalophysum and the anterior extremity of the bulla 

 is rounded and not truncate in the former. The bulla in the species 

 now described presents an angle posteriorly, as viewed from below, 

 instead of the rounded outline seen in several species. 



" The form of the skull differs from that of several species where that 

 region is known. Thus in the C. hurtinii, according to Van Beneden, the 

 occipital bone is broadly rounded in outline instead of triangular. 

 In C. dubium this region is triangular, but is much more elevated and 

 less produced forwards than in the C. megalophysum. It is more 

 elevated than the length from the frontal bone posteriorly, instead of 

 being only two-thirds as high as long. In the C. morenii, from Chubut, 

 Patagonia, Lydekker states that the lateral occipital crests are more 

 elevated than the apex of the occipital bone, giving a cordate outline 

 to the posterior profile. This does not occur in any known northern 

 species. The tympanic bulla of this species is also quite different. 

 The occipital region of the C. hupschii resembles that of the C. 



