Affinities of the Genus Pleuracantlius. 353 



of teeth, attached to the bones of the throat also characterize 

 bony fishes. Against these Teleostean resemblances may be 

 placed others which are again closely related with cartila- 

 ginous fishes, for example, the mosaic-like structure of the 

 integument — though some of the Siluroid fishes are possessed 

 of a rough skin almost like the shagreen of the sharks, and 

 one very curious genus (Sisor) has the whole length of its 

 back mailed ' with enamelled scales. The position of the 

 shoulder-girdle, the suspensory jaw, and the attachment of the 

 upper to the under jaw are similar to those of the Plagiostomous 

 fishes. The spine, situated immediately behind the head, 

 solid and circular or flattened in form, without the fin- 

 investing hollow of the commoner Ctenacanthus or Gi/ra- 

 cantkus, does not resemble the fin-spine of any existing 

 shark ; but, as was pointed out by Dr. Goldfuss, and after- 

 wards insisted on by Prof. Beyrich, it appears to be closely 

 related to the JRays (Squatina) both in form and method of 

 attachment. 



Many of the Siluroid fishes have spines very closely ap- 

 proaching those of Pleuracantlius ; and it may be well to notice 

 one or two examples. Rita Buchanani, Cuv. & Val., is about 

 7'5 inches in length. The head, covered with strong dermal 

 plates, is 1*75 inch across, and very broad and depressed in com- 

 parison with its depth. The mouth is at the termination of the 

 snout ; it is large and has a wide gape ; its jaws are armed with 

 a large number of minute teeth ; and others cover the palatal 

 region of the mouth . The eyes are small. From the head the 

 body of the fish tapers rapidly towards the tail. The skin is 

 scaleless. There are two dorsal fins : the anterior one, situated 

 2*5 inches from the termination of the snout, is armed with a 

 strong articulated spine; the posterior one is an adipose fin 

 without rays. There are two pectoral, a pair of ventral, an 

 anal, and the caudal fins. The pectoral fins are protected by 

 strong bony spines. The features of peculiar interest lie in 

 the spines attached to the anterior dorsal fin and the two pec- 

 torals. The latter are 1*6 inch in length and '2 of an inch 

 broad ; they are somewhat flattened, slightly curved, and 

 end in a point. The upper and lower faces are very finely 

 striated ; and the two lateral extremities are armed with rows 

 of recurved, long, sharp denticles, extending the whole length 

 of the spine, but becoming smaller near the base. The dorsal 

 spine is 2 inches in length, and of about the same diameter as 

 the pectorals ; it is much rounder than the pectorals, and 

 tapers rapidly to a point. It curves slightly backwards, and 

 on its posterior median surface has a single row of denticles 



