228 FISHES CHAP. 



valve, is the spiracular aperture itself. There is a dentigerous 

 splenial on the inner surface of the lower jaw. The hyoid arch 

 has no separate symplectic bone. An operculum and a suboper- 

 culum are present, but no inter-operculum ; and unless the hinder 

 part of the large cheek-plate, which is traversed by the mandibulo- 

 hyoid sensory canal, represents a pre-operculum, the latter is 

 wanting. Branch iostegal rays are absent, but there is a single 

 pair of large jugular plates. 



Very little is certainly known about the cranial cartilage- 

 bones in the fossil members of the group, but the investing 

 dermal bones, which bear a general resemblance to those of 

 Polypterus, are often somewhat more numerous, and they form 

 a very complete dermal armature for the entire head. There is 

 a very complete ring of circum-orbital bones, and very often a 

 ring of sclerotic plates. Two large cheek -plates are often present. 

 Xothing comparable to pre- and post-spiracular ossicles is known, 

 but squamosal and supra-temporals can often be identified. To 

 the ordinary bones of the lower jaw there may be added a series 

 of. infra-dentary plates, and besides the paired principal jugular 

 plates there may also be present a small anterior median plate and 

 a series of small lateral jugular plates on each side, as in the Car- 

 boniferous Rhizodopsis (Fig. 274). Most of the superficial dermal 

 bones, both in the living and extinct Crossopterygii, are invested 

 externally by a granulated or rugose layer of enamel-like ganoin. 



In the Holostei, and especially in Amia, the skull approxi- 

 mates more closely to the normal Teleostean type as represented 

 by the Salmon's skull. In Amia * all the occipital cartilage- 

 bones are present a basi-occipital, two exoccipitals, and a supra- 

 occipital ; and, except for the absence of a pterotic, the periotic 

 series of bones is also complete. Paired ali- and orbito-sphenoids 

 form the lateral walls of the inter-orbital portion of the cranial 

 cavity. Above, the complete cartilaginous roof of the cranial 

 cavity is invested by a shield of suturally united and ganoin- 

 covered dermal plates. The hyomandibular element has a 

 symplectic bone at its distal extremity. There is a complete 

 series of opercular bones, and the branchiostegal rays are 

 numerous. A single median jugular plate is present. The lower 

 jaw has on each side five dentigerous splenial bones in addition 

 to dentary and angular bones, while cartilage-bones are repre- 



1 Sagemehl, Morph. Jahrb. ix. 1884, p. 177. 



