2 



BRITISH FOSSILS. 



fin, and a confused mass of scales and vertebrae. The form of the 

 head varies much in this genus. At Whitby, where the species are 

 more nnmerous than in any other of our British localities, we find 

 a gradation from the small and pointed form of head of Pachy- 

 cormus gracilis and acutirostris to the short and deep head 

 which characterizes Pachycormus latirostris. Professor Agassiz 

 considers the species under notice to be most nearly allied to the 

 latter species, an approximation which is hardly borne out by a 

 more strict comparison of the two than he was enabled to institute 

 from the materials he had at his disposal. The head oi Pachy- 

 cormus latipennis is deeper and broader, and has the snout more 

 blunt than in any other species. It measures four inches in length 

 by two and a quarter in depth ; the breadth across the frontal 

 bones is one inch and a quarter. The lower jaw measures two 

 inches and a half in length from the symphisis to the articulating 

 condyle. It is furnished with numerous conical, incurved teeth, 

 varying in size, the largest being situated on the anterior portion 

 of the jaw. The condyloid process is strong, and articulates with 

 a very broad hypotympanic bone. The upper maxillary bone is 

 more slender than the lower jaw ; it is dentigerous, the teeth 

 corresponding with those of the lower jaw opposed to them. The 

 large laniary teeth of tlie latter probably reciprocated with similar 

 teeth on the premaxillary bone and vomer. The frontal bones are 

 broad and very compact in structure. Their surface is covered 

 with slightly elevated sinuous rugae interspersed with frequent 

 minute punctures, the former prevaling on the posterior, and the 

 latter on the anterior parts of the head. The opercular bones had 

 a somewhat similar surface character, with the addition of a few 

 distant granulations on the upper portion of the operculum. The 

 coracoid bones are very strong, and coarsely plicated longitudinally. 

 The ascending ramus forms nearly a right angle with the lower 

 horizontal branch, this curvature being more abrupt than in 

 other species of the genus. The pectoral fin is very broad, and its 

 constituent rays are flattened as in the genus Sawropsis. It 

 differs from the pectoral fin of Pachycormus latirostris, in being 

 much shorter and less pointed at the extremity. It contains more 

 than twenty rays. Of these the first is very strong, and single ; 

 the succeeding rays remain single for more than half their extent ; 

 they then dichotomize, and show frequent transverse joints. In 

 consequence of the great breadth of the constituent rays the surface 

 of the fin when expanded was very large, a character happily 

 seized upon by Professor Agassiz for the specific definition. The 



