314 FISHES. 



on the abdoainal aspect of the fish ; the body is depressed, 

 and the trunk, which is surrounded by the immensely 

 developed pectoral fins, forms a broad fiat disk, of which the 

 tail appears as a thin and slender appendage. Spiracles are 

 always present ; the number of gill-openings is constantly five; 

 no anal fin ; dorsal fins, if present, situated on the tail. 

 However, some of the Eays approach the Sharks in hav- 

 ing the caudal portion less abruptly contracted behind the 

 trunk. 



Fossil Plagiostomes are very numerous in all formations. 

 Some of the earliest determinable fish remains are believed 

 to be, or are, derived from Plagiostomes. Those which can 

 be referred to any of the following families will be mentioned 

 subsequently : but there are others, especially fin-spines, which 

 leave us in doubt to which group of Plagiostomes their 

 owners had any affinity, thus Onchus from the upper Silurian, 

 continuing to carboniferous formations; Bimeracanthus, 

 Homocanthus, from the Devonian ; Oracainihus, Gyracanthus, 

 Tristychius, Astroptychius, Ptychacanthus, Sphenacanihus, etc., 

 from carboniferous formations ; Leptacanihus, from the coal 

 to the Oolite ; Gladacanthus, Cricacanthus, Gyropristis, and 

 Lepracanthus, from the coal measures ; Nemacanthus, Liacan- 

 thus, from the Trias ; Astracanthus, Myriacanilius, Pristacan- 

 thus, from the Jurassic group. 



A. Selachoidei: Shaeks. 



The elongate cylindrical body, generally terminating in a 

 more or less pointed snout, and passing into a powerful and 

 flexible tail, blade-like at its extremity, gives to the Sharks a 

 most extraordinary power of swimming, with regard to en- 

 durance as well as rapidity of motion. Many, especially the 

 larger kinds, inhabit the open ocean, following ships for weeks, 

 or pursuing shoals of fishes in their periodical migrations. 

 Other large-sized sharks frequent such parts of the coast a,s 



