102 



PISCES. 



often absent on the caudal region (fig. 74), and rarely alto- 

 gether wanting. The best-known genus is 



Mesturus (fig. 73). This fish is entirely covered with rhombic scales, 

 which differ from those of all other known ganoids in being united with 

 each other above and below by jagged sutures. The paired fins are very 

 small, and the caudal fin is not forked. The chondrocranium is well 

 ossified and the basicranial axis is directed downwards and forwards, 



FIG. 72. 



Illustrations of characters of Pycnodont Fishes. , transverse section of jaws, 

 showing the two halves of the manclibular dentition opposing the vomerine 

 teeth; b, vomerine and mandibular teeth of Microdon; c, vomerine and 

 mandibular teeth of Coclodus ; d, portion of vertebral column of Ccelodus, 

 showing persistent notochord (shaded), and the expanded bases of the 

 arches; e, the same of Pycnodus; f, inner view of scales, showing mode 

 of interlocking by pegs and sockets, which are continued as longitudinal 

 ribs. (After J. J. Heckel.) 



making a considerable angle with the axis of the trunk. There is no 

 essified interorbital septum. The parasphenoid is short, not extending to 

 the occiput, while a cartilage bone which seems to be basisphenoid is 

 exposed for a considerable area at the base of the skull. The vomers 

 coalesce. The membrane-bones or dermal plates of the cranial roof form 

 a continuous shield (fig. 73), but their arrangement is not quite constant. 

 The posterior median element (s. occ.) is bilaterally symmetrical and 

 unpaired; the frontals (/.) are the largest plates; a squamosal (sq.) 

 covers the otic region on each side. The "parietal" region is occupied by 

 two or three irregular plates (pa.), though there is only one parietal plate 



