CHIMiEROIDS. 



65 



triangular plates, six times larger than the preceding. The anterior 

 angles of these plates are rounded, and conceal the posterior half of 

 the anterior plates : the inferior broad surface of the anterior teeth is 

 sinuous, and joins the lateral surface at a sharp edge ; the grinding 

 surface of the posterior plates is convex in every direction, and there 

 is a raised portion in the middle of each, of the figure represented in 

 the plate (PI. 28, fig. 1). The upper surface of each of these teeth is 

 concave from side to side, so that it encases or sheaths the alveolar 

 border of the upper jaw, in a manner analogous to the broad teeth of 

 the Cestracion.{l) Both the anterior and posterior dental plates in the 

 upper jaw, meet at the median line of the mouth. The two dental 

 plates of the lower jaw are of a sub triangular form, with the posterior 

 and external sides gently curved ; the broad grinding surface is con- 

 vex on the inner and concave on the outer side ; a trenchant margin 

 divides this from the lateral surfaces of the dental plate. When a 

 longitudinal vertical section is made of these teeth (as in fig. 3, pi. 28), 

 their coarse tubular texture is evident to the naked eye. There is a 

 large pulp-cavity at the posterior parts of both the upper and lower 

 dental plates, and, when the pulp is removed, the exposed surface of 

 the base of the tooth presents a reticulate character from the large arese 

 of the medullary tubes into which the processes of the pulp are con- 

 tinued. These tubes radiate towards the grinding surface of the tooth 

 and dichotomize as they proceed. As these tubes advance towards 

 the surface, their cavity becomes gradually obliterated by calcareous 

 salts, deposited in concentric layers and perforated every where by the 

 minute calcigerous tubes which radiate from the medullary canal ; 

 thus the substance of the tooth increases in density as it approaches 

 the triturating surface. 



A diminished view of the appearance of a longitudinal section of the 

 dental plate of a Chimfsra, as seen under a magnifying power of 400 

 diameters, is given at PI. 29, fig. 1, and of a transverse section at fig. 2. 

 But before proceeding to describe this structure, a few words are requi- 

 site touching the modifications of form presented by the dental plates 

 of Chimcera monstrosa, (PI. 28, figs. 4 and 8), and of some allied ex- 



(1) The genus Cochliodus appears to have been an extinct transitional form between 

 Cestracion and Callorhynchus. 



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