4. PETRORHTNCnUS. 34J6 



elongate, slender, compressed on the sides, fringed on the upper part 

 of the sides by the edges of the enlarged callous intermaxillaries, 

 which contain between them a much-enlarged callous vomer, which 

 tapers in front into the end of the beak, and is truncated behind, 

 filling up the narrowed front i^art of the frontal concavity. 



The upper jaw toothless. The lower jaw slender, produced in 

 fi'ont, toothless ; it may have had two teeth in front in the young 

 state, as there are obscure indications of two pits. 



The skull is much more like the usual form of the skull of the 

 Delphinoid Whales than that of Catodon or Kogia, and somewhat like 

 that of an ILiperoodon without the elevated ridges of the maxillaj on 

 the sides of the beak. 



The peculiarity of the genus is the great development of the inter- 

 maxillaries and the large size and callous state of the upper surface 

 of the vomer. 



The intermaxillary bones which fringe the upper part of the sides 

 of the beak are thick, hard, and shining, formiag with the enlarged 

 vomer the upper part of the beak ; they are expanded behind so as 

 to form the large hemispherical cavity in the crown, with nostrils 

 and blowers at the base of its hinder part. The sides of this cavity 

 are lined internally with the expansion of the intermaxillaries, which 

 are supported on each outer side by a wall formed by the elevation 

 of the inner edge of the hinder part of the maxilla. The wall of the 

 cavity is separated from the outer margin of the maxilla, which 

 forms the inner part of the outer edge of the brain-case, by a deep 

 concavity. 



The upper part of the spermaceti-concavity is arched over by the 

 tliickened prominent nasal bones, and by the dilatation of the thick 

 hinder edge of the walls. 



From the inspection of the drawing by Mr. Trimen of this skull, I 

 was inchned to regard it as a new species of Hyperoodon, forming a 

 peculiar section of the genus, and which I had provisionally named 

 Uyperoodon Capensis (Proc. Zool. Soc. 1865, p. 359) ; but it proved 

 on examination to be an entirely new form, which appears to be in- 

 termediate in structure and form between Hyperoodon and Catodon. 

 It agrees with Catodon and Koy'm in having a large concavity on 

 the crown of the skull, to contain the spermaceti or " head-matter," 

 as it is called by the whalers, above the blowers, and with Hyperoodon 

 in having an elongated beak, with thick prominent nasal bones over 

 the blowers, and in having none or only two or four deciduous teeth 

 in the front of the lower jaw. 



What I believed, in the small drawing made by Mr. Trimen, were 

 the slightly developed lateral expansions of the maxillaries, which 

 are characteristic of the genus Hyperoodon, prove on examination of 

 the skull to have represented the much thickened intermaxillaries 

 and the very large callous prominent vomer which is between them 

 on the uj)per surface of the beak. The skull, as is generally the 

 case in the Cetacea, is considerably distorted, the left side being much 

 the smallest and least developed. 



In Catodon and the allied genus Koyui the spermaceti-cavity 

 occupies the whole upper surface of the skull, and is surrounded by 



