80 FISHES. 



The side of the skull, in front of the operculum, is covered 

 by a large irregularly-shaped bone {T) (corresponding to the 

 "tympanic lamina" of Geratodus, Fig. 35, q), held by some to 

 be the prseoperculum ; along its upper circumference lies a 

 series of small ossicles, of which two may be distinguished 

 as spiraculars, as they form a valve for the protection of the 

 spiracular orifice of these fishes. An infraorbital ring is repre- 

 sented by a prse- and post-orbital only. 



Each hyoid consists of three pieces, none of which bear 

 branchiostegals, the single median piece being osseous ia front 

 and cartilaginous behind. Four branchial arches are developed, 

 the foremost consisting of three, the second and third of two, 

 and the last of a single piece. There is no lower pharyngeal. 

 Between the rami of the lower jaw the throat is protected by 

 a pair of large osseous laminee (gular plates), which have been 

 considered to represent the urohyal of osseous fishes. 



The scapulary arch is almost entirely formed by the well- 

 developed membrane-bones, which in the ventral line are 

 suturally united. The pectoral fin is supported by three 

 bones, pro-, meso-, and metapterygium, of which the dUated 

 middle one alone bears rays, and is excluded from the articu- 

 lation with the shoulder-girdle. 



The pubic consists of paired bone, to which tarsal bones 

 supporting the fin-rays are attached. 



In the Lepidosteoidei the vertebras are completely ossified, 

 and opisihoccelous, having a convexity in front and a concavity 

 behind, as in some Amphibians. Though the end of the body 

 externally appears nearly diphycercal, the termination of the 

 vertebral column is, in fact, distinctly heterocercal (Fig. 40). 

 Its extremity remains cartilaginous, is turned upwards, and lies 

 immediately below the scutes which cover the upper margin 

 of the caudal fin. It is preceded by a few rudimentary verte- 

 brae which gradually pass into the fully developed normal 

 vertebrae. The caudal fin is suspended from hajmapophyses 



