SHARKS. 31 



finely serrated. (1) This description applies to the lateral rows of teeth 

 in which the points are inclined backwards and outwards ; the teeth 

 of the mesial row in both jaws are of a symmetrical figure. The 

 teeth are alike in the upper and lower jaws. 



There is the same correspondence, as regards the upper and 

 lower jaws in the singularly formed teeth, which characterize the 

 Squalus spinosus of Schneider, — the type of the genus Goniadus of 

 Agassiz (PI. 4, fig. 4). In these teeth, the point is so far inclined 

 backwards, that the anterior margin forms a nearly horizontal 

 trenchant edge, applied to the corresponding margin of the opposite 

 tooth : two denticles project horizontally forwards from the base of 

 the anterior margin, and one or two similar denticles from the oppo- 

 site side of the tooth. 



In the genus Scymnus, (PI. 4, fig. 3,) the teeth of the lower differ so 

 much from those of the upper jaw, that nothing save actual in- 

 spection of the jaws in situ could lead to the belief that they be- 

 longed to the same animal. The teeth of the upper jaw are small, 

 conical, subcompressed, with slightly recurved points ; those of the 

 median or anterior rows present a nearly subulate form : the poste- 

 rior ones are somewhat broader : their osseous base is bifurcate, as in 

 Lamna. The teeth of the lower jaw are about eight times larger than 

 those above ; they are straight, flattened, symmetrical, lancet-shaped 

 plates, with finely dentated margins. The base of the tooth is bifur- 

 cate, the divisions being parallel, and divided by a fissure which becomes 

 slightly dilated near the enamelled crown. The middle tooth of the series 

 rests upon the symphyseal line, and has one basal fork attached to 

 each ramus of the lower jaw ; its parallel lateral edges overlap those 

 of the contiguous teeth; the posterior edge of each lateral tooth 

 overlaps in a similar manner the anterior one of the tooth behind : 

 this imbricated disposition is quite peculiar to the Scymnus, among 

 sharks. There are four recumbent teeth in each vertical row, the 

 apices of which are turned down in a direction diametrically opposite 

 to that of the erect exterior tooth. The shedding of the outer teeth 

 appears to be simultaneous, and the change of position of the succeed- 

 ing series must be very rapid, as well as extensive. 



(1) PI. 28, fig. 9. 



