SAUROIDS. 75 



anchylosed to the jaw-bone when their growth is completed ; and, as 

 in most other fishes, the succession of new teeth seems to be uninter- 

 rupted. 



The extinct genera of this family were much more formidably 

 armed, and the great conical laniary teeth of some of the individuals 

 compete in size and strength with those of the largest Ichthyosaurs 

 and Crocodiles. Plate 35, fig. 2, is a reduced figure of the dislo- 

 cated and fractured premandibular bones of an extinct sauroid 

 species of the genus Rhizodus, a genus nearly allied to the Holop- 

 tychus of Agassiz, but differing in the greater number, and more 

 robust and obtuse shape of the smaller conical teeth. In each pre- 

 mandibular bone there are three elongated conical teeth, with several 

 smaller and more obtuse conical teeth in the interspaces. 



The larger teeth have an ovate transverse section, with a trenchant 

 posterior margin, and terminate above in a sharp point ; they are 

 thus alike fitted for piercing and cutting. Their base is irregularly 

 fluted in the longitudinal direction, and sinks deep into the substance 

 of the jaw, with which it is firmly anchylosed. The peculiarly 

 efficient mode in which these large destructive teeth are implanted 

 in the jaws, indicates the violence and force with which they were 

 wielded in the predatory contests of the living fish. Fig. 1,P1. 36, 

 shows the external form of one of the larger teeth of the Rhizodus. 

 A longitudinal and vertical section has been removed from the 

 grooved base, showing its solidity, and the coarse longitudinal fibrous 

 structure which it presents to the naked eye. Fig. 2 exhibits the 

 complicated organization which a section of a portioQ of the basis of 

 the tooth presents under a magnifying power of ^ inch focus. 



The exserted body of the tooth is hollow, as in other Sauroids, 

 but the pulp-cavity is relatively smaller ; the parietes of this cavity 

 consist of a dense ivory, composed of minute calcigerous tubes. The 

 diameter of these tubes at their origin is ^^th of a line. They proceed 

 in slight curvatures from the central canal at right angles to the 

 periphery of the tooth, with interspaces equal to four of their diameters ; 

 throughout their course they are minutely undulated, and occasionally 

 divide dichotomously ; they give off ramuscules at very acute angles, 

 which are lost in the clear interspaces. The thin external enamel- 



