74 



BRITISH FOSSILS. 



Description. — On separating all the specimens of Cheimcanthus 

 from the other genera with which they were associated it was 

 evident that they indicated two distinct forms, one with a long 

 tapering body and the fins rather distant from each other, the 

 other short and thick, and having the fins closely approxi- 

 mated. The former is probably identical with Gheiracanthus 

 micTolepidotus, so common at Lethen ; the latter I consider to be 

 new. Both species appear to have been very abundant in the 

 Tynet locality and to have been gregarious, but Cheimcanthiis 

 latus seems to have been most numerous. The collection forwarded 

 to me contained above fifty examples, more or less perfect, of this 

 fish. The length of an average-sized specimen is about six inches 

 from the nose to the extremity of the tail, and the depth in front of 

 the ventral fins one inch and a half, being an excess of one quarter 

 of an inch as compared with a specimen of Gheiracanthus microle- 

 pidotus of similar length. The head is seen in profile, and measures 

 an inch and a quarter in length from the point of the lower jaw to 

 the posterior edge of the operculum. The moutli is large and 

 horizontal ; the teeth are not preserved in any of the specimens. 

 Judging from the character of the dentigerous bones they were pro- 

 bably small and in single rows. The branchiostegous rays are very 

 numerous, and extend high up in the opercular space. They are 

 composed of a harder substance than the surrounding parts, being 

 almost as dense as the fin spines, and are consequently preserved in 

 most of the specimens. The pectoral fins are large and triangular. 

 They are supported b}^ a pair of strong spines, slightly curved and 

 measuring one inch and a quarter in length. These are attached to a 

 pair of coracoid bones, broad at the point of articulation and tapering 

 upwards. A short process extends downwards from the same point 

 on either side, meeting its fellow in the median line, and completing 

 the thoracic arch. The large expanse of the pectoral fins required 

 a powerful fulcrum such as this to enable them to fulfil their 

 functions. The ventral fins are situated midway between the pec- 

 toral and anal fins. The spines supporting them are straight, 

 and have their bases inserted in the integuments without any trace 

 of pelvic bones. The dorsal fin occupies the middle of the back, the 

 point of insertion of the dorsal spine being opposite the interspace 

 between the ventral and anal fins. It is a large triangular fin 

 attached to a long and straight spine deeply implanted in the 

 muscular tissue. The anal fin spine occurs midway between the 

 ventral spines and the base of the caudal fin, the membranous 

 portion extending as far as the anterior margin of the latter organ. 



