OSSEOUS SYSTEM. 203 



of the Sturgeon as in those of the Osseous Fishes ; in the Sharks 

 and Rays several divisions of triangular and quadrangular cartilaginous 

 laminae occur ; they correspond to the accessory spines and support 

 the fin-rays. 



Many of the Cartilaginous Fish, as the Rays, Sharks, Sturgeon, 

 and the genus Lepidosiren, are provided with Ribs. The Cyclostomi, 

 at least the Lampreys, possess a peculiar sternal series, formed of a 

 number of ramified rib-shaped cartilages which are united together, 

 and to an elongated sternoid cartilage ; it encloses the gills, and may 

 be therefore most properly compared with the branchial skeleton of 

 other Fishes. 



Among the Cartilaginous Fishes we meet with an entire absence 

 of Extremities in the Cyclostomi, while in the Sturgeons, Chimaerae, 

 Rays, and Sharks, they exist in the form of pectoral and ventral 

 fins. The anterior extremities of the Sturgeon resemble those of the 

 Osseous Fishes in being composed of several pieces, which correspond 

 to a scapula and clavicle ; the posterior extremities are a pair of small 

 ventral fins. The Chimaera approximates most in this respect to the 

 Plagiostomi. 



Among the Plagiostomi the Rays present us with a surprising 

 development of their pectoral Jins, which correspond to anterior ex- 

 tremities, and are in some degree analogous to those of the Osseous 

 Fishes. Their first portion consists of a scapular and a clavicular 

 cartilage ; frequently, however, of three cartilages that form a pretty 

 broad arch, which in the Rays firmly unites with the anterior anchy- 

 losed section of the vertebral column, but does not reach the latter 

 in the Sharks. To this first portion succeeds a second, which con- 

 sist in the Rays of three or four very elongated cartilaginous 

 pieces ; next we meet usually with two rows of rays, the posterior 

 of which may be perhaps likened to the metacarpus, while the an- 

 terior or external represents as fin-rays the digital phalanges. In 

 the Rays the digital phalanges are somewhat dilated at the two 

 ends. 



In the Plagiostomi and Chimaerae the arrangement of the posterior 

 extremities is rather more perfect than in most Osseous Fishes. An 

 iliac cartilage is here present as a rudiment of the pelvis, and to 

 this follows a series of more elongated cartilages, which may be re- 

 garded as tarsal, while arranged upon these are the rays or pedal 

 phalanges supporting the ventral fins. In the male Rays and 

 Sharks a pair of long, slender, cartilaginous appendages are united 

 to the iliac cartilage ; they are hollowed out by a groove above for 







