270 



HARDWICKE'S SCIENCE-GOSSIP. 



numerous ichthyodorulites of the Carboniferous 

 Limestone may be safely associated with the teetli, 

 and very perfect jaws from American strata show 

 that even our admiral:)le specimens from the Bristol 

 quarries are exceedingly incomplete examples. Every- 

 one acquainted with geological text-books is familiar 

 with the figure of the mandible of CocltUodus contorlus 

 (fig. 173), which exhibits three inrolled teeth on each 

 ramus, and it is not difficult to perceive how these 

 few sijigle teeth represent the successive transverse 

 scries of teeth in the living Ccstracio7i,—i\\Q type of 

 the Cestraciontida;. As in the latter family, the new- 

 dental substance is added at the inner side (y), and 

 growth thus proceeds from the inside, outwards ; but 

 while, in Cestracion, the worn-out teeth of the outer 

 border fall away as soon as they are no longer required, 

 such cannot take place in Cocldiodns without fracture, 

 and to avoid this the border becomes inrolled. 



jjossess median or symphysial teeth that were once 

 regarded as species of Hclodns. Pscpliodiis is one of 

 the most interesting, and, although no specimens 

 exhibiting the actual arrangement of the teeth has 

 hitherto been described; a study of the large series in 

 the collection of the Earl of Enniskillen has led Mr. 

 J. W. Davis to suggest* that each side of the jaw 

 was provided with a row of three The teeth them- 

 selves are much (fig. 175) flatter than those of Coch- 

 liodus and very variable in shape ; some exhibit a 

 slightly concave surface, while others, equally worn, 

 are distinctly convex, and this appears to show that 

 the dentition of the upper jaw was very similar to 

 that of the lower, the only difference being in the 

 convexity or concavity of the crushing surface. Hc- 

 lodns plaints, Agass., is definitely proved to belong 

 to Fsephodus magmis, but it is uncertain at present 

 what part of the mouth the teeth of this form occupied. 



Fig. 161 . — Prisficladihiiis 

 dentatus. 



Fig. 162. — Cladodus 

 rniialilis. 



Fig. 163. — Hyhodus 

 ZrossicoHus. 



Fig. 164. — Cephalic spine of HyboduE. 

 a, outline of base. 



Fig. 165. — Hybodiis (dorsal fm spiiie). One-tliird natural size. 



Fig. 166. — Hybodiis 

 rcticulatiis. 



Fig. zQi-j. — I^yhodiis 

 Dctahecliei. 



Fie 



168. — Ilyhod'us 

 mcdiits. 



^!ms^^^^^ 



Fig. 169. — Orodus ramosus. 



forming quite a small spiral in the older individuals, 

 as shown in the diagrammatic section, (fig. 173 a). 

 These arc two of the most remarkable peculiarities of 

 the genus now under consideration, and there is also 

 <me other to which Sir Richard Owen attached great 

 imj)ortance when he established the Cochliodont 

 family, namely, that the symphysial area (x) appears 

 totally destitute of teeth, whereas the corresponding 

 part of Cestracion is armed with dentition of a 

 prehensile type. This conclusion, however, has been 

 invalidated by more recent researches in America, 

 anil some beautiful specimens from the Carboniferous 

 Limestone of the United States, leave no doubt that 

 the symphysis was occupied with teeth of the kind 

 commonly known as Hclodns when Ihey happen to be 

 found detached. 



The other" genera of Cochliodontidre differ from 

 the Cestracionts in the same way as Cochliodus, — in 

 the large transverse teeth, and in the inrollment by 

 growth, — and some have been proved, likewise, to 



Another characteristic genus is Dcltodns, in which 

 the upper teeth are known to differ very much from 

 the lower. The imperfect specimens hitherto met 

 with, seem to show that the upper jaw was only 

 armed with two dental plates, of the form rcjiresented 

 in fig. 176, while the lower possessed at least two, and 

 probably three, in each ramus. The latter (fig. 174) 

 are not so "deltoid" as the former, being much 

 rounder and convex, and exhibit very distinctly the 

 inrollment of the outer border. Sandalodns is a 

 closely allietl geiuus, represented in Britain by the 

 large species S. dlfoi-risil (Davis), from the Carboni- 

 ferous limestone of Oreton and Bristol, and 

 particularly remarkable because the lower jaw appears 

 to resemble the upper in not being provided with 

 more than two of the inrolled teeth. 



Plcurodiis is interesting in consequence of the 

 structural details revealed by a specimen discovered 



* " Trans. Roy. Dub. Soc." Ser. 2, Vol. I, pp. 416, 417. 



