REMAINS OF REPTILES AND FISHES. 117 



this genus in the "Poissons Fossiles"* shows "at a glance" 

 that it is generically distinct from Ageleodus ; and it is certainly 

 erroneous : the difference is merely a difference in degree. The 

 medullary canals are more elongated and somewhat more regu- 

 larly parallel in Agassiz's figure than they are in our specimens, 

 in many of which, however, the parallel and elongated character 

 predominates. In fact, there is quite as great a difference in 

 this respect between individuals of our suite of specimens as 

 there is between some of them and Agassiz's figure referred to. 

 And it must not be forgotten that this figure represents the struc- 

 ture in a different species. We repeat, then, that no generic dif- 

 ference is perceptible at a glance. M. Agassiz certainly states 

 that the substance at the base of the tooth is perfectly homoge- 

 neous. In some of our specimens, too, the basal portion has lost 

 nearly all traces of structure ; but such specimens are mounted 

 in balsam, which, we have seen, is liable to render minute struc- 

 ture invisible. It is therefore not improbable that the specimens 

 of M. Agassiz may have been mounted in this medium ; and it is 

 equally likely that the minute structure was not preserved in the 

 fossil examined by him. Such discrepancies must be expected 

 in the examination of fossils ; and accordingly we have already 

 seen that the minute structure in Ctenodus had escaped the ob- 

 servation of that naturalist. 



In Ageleodus we see another striking instance of the danger 

 of trusting entirely to the sections of objects not previously un- 

 derstood. From this cause the denticles are described as if their 

 whole contour was seen, whereas there is nothing but the mere 

 stumps left in the section, the crowns all having been cut away 

 in making it. As the denticles are (as we have aheady stated) 

 recurved, they must necessarily, to a great extent, be removed 

 in such a section as that figured. Had this been j)reviously 

 known, the bases of the denticles could never have been mis- 

 taken for their crowns, nor could the latter have ever been de- 

 scribed as "broader than they are high;" nor could it have 

 been stated that they all "terminate obtusely; and this seems 

 to be an original form, not due to wear or abrasion." In fact, 



*Tome III., plate M.. fins. 1, 0. 



