158 PISCES. 



thereby a rough shagreen-like surface. Sometimes, especially on 

 the tail (Raiidce), there are larger bony plates, arranged in rows and 

 provided with pointed spinous processes, which serve for protection 

 (ichthyodorulites). All the Selachians have large pectoral and 

 pelvic tins. The former are attached by a cartilaginous shoulder- 

 girdle to the posterior part of the skull, or to the anterior region of 

 the vertebral column ; they are either sharply marked off and have 

 an almost vertical position on the anterior part of the body (Ghimcera 

 and Squalidce), or they have the form of very large, horizontally- 

 placed lateral expansions of the body (Rays]. In the latter case 

 they reach by means of the so-called cranial fin cartilages to the 

 anterior end of the snout, and lean by posterior suspensors on the pelvic 

 framework of the ventral fins ; the latter are always placed near the 

 anus, and in the male bear peculiar grooved cartilaginous appendages, 

 which are the accessory copulatory organs (claspers}. The unpaired 

 fins also may be well developed, and, as their number and position 

 varies in the different forms, they may be of systematic importance. 

 A sharp bony spine is sometimes present in front of the dorsal fins, 

 or completely isolated on the dorsal surface of the tail (Tryyoii), and 

 this as well as the spinous and hooked processes of the dermal 

 bony plates serve as a weapon of defence. The caudal fin is always 

 markedly heterocercal externally. 



The skull is an undivided, cartilaginous capsule, the base of which 

 sometimes is articulated to the vertebral column (Chimcera and 

 Raiidce), while sometimes it is excavated like the body of a vertebra 

 (fig. 571). On the facial region the cartilaginous mandibular arch 

 persists, and is attached to the auditory region of its skull by the 

 hyomandibular. The palatoquadrate bar is moveably connected to 

 the cranial capsule (except in Chimcera). The palatoquadrate and 

 the lower jaw are always cartilaginous, and as a rule are abundantly 

 furnished with teeth. The vertebral column with its remains of 

 the notochord is also principally cartilaginous, but separate biconcave 

 vertebrae are developed, the form of which offers numerous variations. 



In all cases there are dorsal and ventral arches, which sometimes 

 remain separate and sometimes fuse with the vertebral bodies. Ribs 

 only appear as cartilaginous rudiments. 



In the structure of the gills (fig. 593), the Selachians differ essenti- 

 ally .from the Teleosteans in possessing five branchial pouches on 

 either side ; the branchial lamellae are attached in their whole length 

 to the partition walls, which are supported by the lateral cartilagi- 

 nous rays of the branchial arches. The branchial pouches are placed 



