198 PISCES. 



occurs in the Cyclostomi. The cartilaginous corpuscules disappear 

 by degrees, and the cartilage appears devoid of cells, like the hairs 

 of one of the Goat tribe. 4th. The completely-ossified cartilaginous 

 tissue, as exemplified in the solid vertebral bodies of the Rays and 

 Sharks. 



The Sturgeon, although a true Cartilaginous Fish, effects the 

 transition from that type to the Osseous, of which we are reminded 

 by the disposition of the parts of its skeleton. Already do we find 

 in many of the Osseous Fishes, as the Pike and Trout, that the 

 cranium consists internally, where it encloses the brain, of cartilage, 

 upon the external surface of which the bones of the cranium, as 

 already described, are deposited. In the Sturgeon the cartilaginous 

 cranium becomes ossified at its base, this bony portion correspond 

 ing to the basilar element or body of the occipital and sphenoid. 

 But even here the ossific process has taken place only externally, 

 and that surface which forms the cavity of the cranium still contin- 

 ues cartilaginous ; the ossification is most apparent in the fibrous 

 tunic, which, continued from the fibrous sheath of the medulla spi- 

 nalis, lines the skull. In other parts the cranium is perfectly carti- 

 laginous, and covered in with firmly adherent bony scutes, which 

 belong properly to the dermo-skeleton, and admit only of an inaccu- 

 rate or very remote comparison with the cranial bones of the Osseous 

 Fishes. In front and on the sides the cartilage of the head presents 

 depressions for the eyes and nasal cavities, and then projects into a 

 long process. The palatal bones are separate from the skull, united 

 with the superior maxilla, and consists posteriorly of an azygos bony 

 plate, and two anterior osseous pieces in pairs. The quadratal bone 

 consists of a superior bony piece united to the cranium and two infe- 

 rior cartilaginous pieces ; the last of these is united to the lower 

 jaw, which, with the small and double superior maxillary bone, 

 closes the edentulous mouth. 



In the Chimaera, the Sharks, and the Rays, the skull is a large 

 cartilaginous capsule, enclosing the brain and auditory apparatus ; 

 it is free of the vertebral column, and exhibits no traces of ossifica- 

 tion. In the Plagiostomi there usually remains upon its upper sur- 

 face an open space or fontanelle, closed only by a fibrous membrane, 

 This cartilaginous capsule is particularly flattened upon its upper 

 surface in the Rays, and has posteriorly an opening, the occipital or 

 foramen magnum, and lesser apertures also for the exit of nerves. 

 A portion of this capsule, convoluted upon itself laterally, forms the 

 orbitar cavities, and frequently presents superiorly, as in Scymnus 



