278 ENALIOSAURS. 



prominent, yet they are less slender than in the Ich. tenuirostris.*' 

 They are also much fewer in number ; I have not found more than 

 forty teeth on each side of the upper jaw, or than thirty-five on each 

 side of the lower jaw. 



In the Ich. acutirostris the teeth differ from those of the Ich. 

 tenuirostris in having a somewhat wider base in proportion to their 

 length : in the specimens which I have examined the teeth likewise 

 present less regularity of size, and they are fewer in number ; in a 

 fragment of jaw, three inches in length, which included twenty-four 

 teeth, their exserted crowns presented a regular alternation of three 

 and five lines in length. In an entire head there are about fifty teeth 

 on each side of the upper jaw, and about forty teeth on each side of 

 the lower jaw. 



I have investigated the microscopic structure of the teeth of the 

 Ichthyosaurus in the si^tecies platyo don 2ind intermedins {I). 



The dentine has the same simple compact structure as in the 

 teeth of existing carnivorous Saurians. The calcigerous tubes present 

 a diameter of si^oth of an inch, with interspaces of g^gth of an inch. 

 They radiate from the pulp-cavity, and form a line continued from 

 its upper end to near the apex of the tooth, according to their usual 

 course, towards the periphery of the tooth ; they describe at their 

 origin a graceful curve, the concavity of which is directed towards the 

 base of the tooth, and then proceed in straight lines at right angles to the 

 periphery of the tooth(2). The secondary curvatures or undulations 

 of the tubes are more regular, more numerous, and more marked than 

 in the crocodile's tooth ; the tubes divide dichotomously many times 

 during their course, and send off lateral branches obliquely into the 

 clear intermediate substance, and principally from their concave side ; 

 the terminal divisions of the calcigerous tubes {a, fig. 2) become less 

 regular, and appear to decussate and communicate at their extremities, 

 either directly with one another by inosculating loops, or through the 

 medium of minute cells. 



The enamel (h) is a clear dense substance presenting faint traces 



(l)Trans. Brit. Assoc. 1838, p. 144. 



(2J The disposition of the calcigerous tubes, as seen in a longitudinal slice of the apical part 

 of the crown of the tooth, is figured in Plate 73, A. 



