66 CHIMiEROIDS. 



tinct species which present the same intimate structure of the dental 

 substance. 



In the Chimcera monstrosa, the dental plates differ considerably in 

 their general form and disposition from those of the Chimcera or Callo- 

 rhynchus australis ; they are extended in the vertical more than in the 

 horizontal direction, sheath the exterior of the jaws, and serve as in- 

 struments for cutting and dividing rather than for triturating and 

 crushing. The anterior teeth of the upper jaw, more especially, pre- 

 sent characters of which we find no trace in the teeth of the last 

 described recent species ; the dental substance being so arranged as to 

 resemble a series of oblique superimposed lamellae. The fossil dental 

 plates of the extinct species of Chimcera, discovered and described by 

 Dr. Buckland, resemble the northern rather than the southern Chimsera 

 of the present day ; some of these teeth, as those of ChimcEra Colei, and 

 Chim. Owenii, resemble in size, though they differ in form from the 

 dental plates of Chimcera monstrosa, but the dental plate of the lower 

 jaw of the Chimcsra Townsendii, which is upwards of six inches in 

 length, indicates a species which, ceteris paribus^ must have surpassed 

 eight feet in length. 



In the marine eocene formation called the London clay, at Sheppey, 

 the dental plates and portions of the jaws of an extinct Chimara, three 

 or four times the size of the existing species, are occasionally found ; 

 the laminae of the anterior tooth of the upper jaw are bent obliquely, 

 so that the outer margin of the plate presents a series of lines, which 

 are horizontal on its anterior part, and bent obliquely upwards at the 

 side. The posterior dental plate has its outer wall composed of a 

 series of vertical columnar portions, the lower ends of which, as they 

 are worn by attrition, cause the trenchant margin of the jaw to be 

 notched and irregular. The inner surface of the same plate is 

 convex, smooth and punctate; it encloses, as it were, a portion of the 

 jaw itself. 



In the extinct Chimaeroid subgenera Edaphodon and Passalodon, 

 discovered and so called by Dr. Buckland, the teeth are long, 

 broad, and thick, with surfaces adapted for crushing and bruising, 

 like the dental plates of Callorhynchus ; but, instead of encasing the 

 alveolar borders of the jaws, they are themselves inclosed in the sub* 



