FISHES CHAP. 
to 
N 
~ 
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. Branchiostegal 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. 
Nothing 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. 
