from the Shales of the Northumberland Coal-feld. 375 



tion ; and the lower margin is often produced into two or more 

 fang-like processes. 



In the base of each denticle there is a small pulp-cavity 

 that extends onlj a short way upwards, and is in direct com- 

 munication with the wide medullary canals of the basal por- 

 tion, which are for the most part elongated ; but in this respect 

 there is considerable variation. The canals are most elongated, 

 as might be expected, in elongated specimens. The dentinal 

 tubules, which are nearly vertical, are coarse, fasciculated, and 

 much branched ; and the osteo-dentine of the base exhibits 

 also a few branched tubules, strongest and most numerous 

 above and at the margins; below they are comparatively 

 small and obscure. 



A few specimens have occurred which are much elongated 

 transversely, and have upwards of twenty denticles ; these are 

 probably C. denticulatus of Agassiz. Ctenoptychius is pro- 

 bably a dermal tubercle, though it certainly has more the ap- 

 pearance of a tooth than either Diplodus or the spined dermal 

 tubercles which have been assigned to Gyr acanthus. 



Note. — That Ageleodus diadema of Prof. Owen (pi. 4) is 

 the fossil above described cannot for a moment be doubted. 

 In general form, size, number and character of the denticles, 

 as seen in section, all exactly agree ; and there is no difference 

 whatever in the histological features : only the specimen 

 figured and described in the paper referred to is shorter than 

 usual ; hence the medullary canals are not so decidedly elon- 

 gated as they frequently are. Now no paleeontologist would 

 hesitate to pronounce our specimens to be Ctenoptychius pecti- 

 natus of Agassiz. It is therefore futile to assert that the figure 

 of the structure of 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 dif- 

 ference in degree. The medullary canals are more elongated 

 and somewhat more regularly 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 structure in a different species. 

 We repeat, then, that no generic difference is perceptible at a 

 glance. M. Agassiz certainly states that the substance at the 

 base of. the tooth is perfectly homogeneous. In some of our 

 specimens, too, the basal portion has lost nearly all traces of 

 * Tome iii. pi. M. figs. 4, 5. 



