2 



BRITISH FOSSILS. 



Description. — Lord Enniskillen's specimen, the only one hitherto 

 found, measures ten inches in length. The parts anterior to the 

 eye socket and behind the termination of the anal fin are deficient. 

 The depth is nearly uniform from the nape to the anal fin, being 

 about two inches and a half Behind the anal fin, the depth is 

 one inch and a half From the gradual convergence anteriorly 

 of the outlines of the head, it is probable that the muzzle was 

 elongated, after the fashion of an JEugnathus or Sauropsis. The 

 gape was large, and both jaws were furnished with numerous sharp 

 conical teeth. The branchiostegous rays are flattened, and ap- 

 parently not so numerous as those of Pachycormus. Twelve are 

 distinguishable, of which the middle ones are the broadest. The 

 opercular flap is crescentic in form, and extends some distance 

 behind the line of union of the vertebral column with the occiput. 

 The preoperculum is strong and prominent, and is invested with 

 a thick coat of ganoine, plicated longitudinally. The surface orna- 

 ment of the operculum and sub-operculum is finer and more granular 

 in its arrangement, and does not conceal the lines of successive 

 growth running parallel to the free margin of the flap. The pec- 

 toral fin is broken ofi* at a short distance from the base. It con- 

 tains about twenty-six rays ; these, as far as they are preserved, 

 are single, without transverse sutures. The rays of all the other 

 fins correspond in being compressed and imbricate. The ventral 

 fin has a few short anterior rays, and about twenty principal ones. 

 They are so closely crowded together, that it is impossible to dis- 

 tinguish the precise number. The rays of the dorsal fin are grace- 

 fully curved backwards ; they are in number about twenty-four. 

 The transverse joints are at distant intervals. The anal fin is 

 remarkable for its extension towards the tail ; it measures one 

 inch and three quarters along the base, and probably contains 

 between fifty and sixty rays. The squamation of Thrissonotus is 

 very elegant, with regard both to the shape and beauty of the 

 component parts, and the graceful arrangement of the dorso-ventral 

 lines. The scales are all comparatively small, but vary much in 

 relative size. The largest, as is generally the case, occur in the 

 neighbourhood of the lateral line, and the smallest, on the abdominal 

 region. The former are rhomboidal in form, the latter are long and 

 narrow, resembling elongated parallelograms. The surface orna- 

 ment is, for the most part, due to the permanence of the successive 

 lines of growth ; but, in the anterior part of the body, and 

 especially in the vicinity of the nape, a few small vermicular pro- 



