52 



BRITISH FOSSILS. 



quarter inches, the dimensions of the tail supplied by the second 

 specimen, and three-quarters of an inch for the absent portion of 

 the head, the entire length of the fish will be ten and a half inches. 

 The depth at the greatest diameter is two inches. The body is 

 irregularty fusiform, the dorsal line being less curved than the ventral 

 outline, and the anterior half of the body more obtuse than the 

 caudal portion. The bones of the head (with the exception of a 

 small fragment of the operculum) are wanting, but the impressions 

 distinctly left upon the matrix show that they were sculptured in 

 rather a bold pattern, not. unlike the ornament on the cranial 

 bones of some of the Holoptychii, and consequently dilFering in 

 this respect from the corresponding parts in Dipterus. The pectoral 

 fins are very indistinctly seen. They appear to have had a short 

 obtuse lobe forming the base, and extending therefrom a set of 

 numerous fin-rays more elongated than those forming the pectoral 

 fin in Dipterus. The small anterior dorsal fin is situated at the 

 commencement of the last third of the body, and is opposed to the 

 ventral fins. The latter are broad and composed of numerous rays 

 expanding from a short lobate base. Both these and the pectoral 

 fins difi'er from the corresponding organs in Dipterus in having 

 more numerous and longer rays. 



The structure of the other fins is very singular and requires 

 a more detailed description. The second dorsal fin is placed 

 immediately opposite the anal fin, and resembles it so, closely 

 that one description will serve for both. In each of these fins 

 the component rays spring from three interspinous bones, and these 

 are attached to a single broad spinous apophysis. The latter 

 bone is probably a composite one, formed by the union of three 

 or more spines. The interspinous osselets have C3dindrical shafts 

 expanded at each extremity, the one for attachment to the 

 vertebral spine, the other for affording a base for the insertion of the 

 fin-rays. In the anal fin the anterior bone of the triplet is shorter 

 than the others and than the corresponding bone in the dorsal fin. 

 The fin-rays springing from the first bone are the strongest. The 

 anterior ones are the shortest and they lengthen in succession until 

 the maximum extent of the fin is attained. They are single at first, 

 hv.t bifurcate in the distal part of the fin. The transverse joints 

 are numerous. The group abutting upon the first interspinous 

 bone contains about six rays. The second bone carries about 

 eight rays, more slender than the former and more frequently 

 sul^divided. The anterior ray of this group is tlie longest, the 

 .subsequent ones decreasing in length in succession. The third 



