A< TINoI'TKKYMI. 



91 



Chondrosteus (figs. 66 68). The trunk is elongate-fusiform and the 

 tail heterocercal. Scales are entirely absent, except on the upper caudal 

 lobe, where they are thick and rhombic, invested with ganoine. The 

 snout must have l>cen prominent, the eye is far forwards, and the toothless 

 mouth is small and inferior. Nothing is known of the ossifications in the 

 chondrocraniurn, but many specimens exhibit the chief membrane-bones. 

 Those of the cranial roof form a continuous shield, without a median 

 longitudinal series of unpaired elements (fig. 66), and comprise the 

 ordinary pairs of parietals (p.), frontals (/.), postfrontals (/>./.), and 

 squamosals (sq.} ; while immediately behind there is a regular row of 

 supratcmporal plates (s.t.). The rostral plates (if any) are unknown ; and 



FIG. 66. . 



Cltondrosteits acipenseroides ; portion of cranial roof and adjoining plates, 

 reduced in size. L. Jurassic (L. Lias) ; Dorsetshire and Leicestershire. 

 /, frontal; op., operculum ; p, parietal; p.f., postfrontal ; p.t., post- 

 temporal; s.t., supratemporal ; sq., squamosal. (After Traquair.) 



there appears to be only one suborbital cheek-plate (fig. 67, s. o.) imme- 

 diately behind and below the eye. The hyornandibular (h.m.) is shaped as 

 in modern sturgeons, but apparently ossified as far as its lower extremity ; 

 and a symplectic must have been present, although it is doubtful 

 whether this was ossified in any part. The pterygo-quadrate arcade 

 exhibits only two ossifications on each side, one being a large expanded 

 element of pterygoid nature, and the other a small v-shaped bone articu- 

 lated with the maxilla, which may be either palatine or ectopterygoid. 

 There is a long curved maxilla (>.<.), meeting its fellow of the opposite 



