116 MESSES. HANCOCK AND ATTHEY ON 



where it is strongly defined from the base by a deep transverse 

 constriction. A lateral section conseqently presents a sigmoid 

 curve, the lower member of which is the larger and less bent. 

 The whole of the denticulated margin, including the denticles, 

 is coated with a thin layer of enamel, only traces of which can 

 usually be seen in sections. The base narrows suddenly imme- 

 diately below the denticulated margin, and is frequently consi- 

 derably longer than the upper glazed or enamelled portion ; 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 only a short way upwards, and is in direct communica- 

 tion with the wide medullary canals of the basal portion, which 

 are for the most part elongated ; but in this respect there is con- 

 siderable 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. denticidatus of Agassiz. Ctenoptychius is probably 

 a dermal tubercle, though it certainly has more the appearance 

 of a tooth then 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 ge- 

 neral form, size, number, and character of the denticles, as seen 

 in section, all exactly agree ; and there is no difference what- 

 ever 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 elongated as they 

 frequently are. Now no palaeontologist would hesitate to pro- 

 nounce our specimens to be Ctenoiyty chins jyectinatus of Agassiz. 

 It is therefore futile to assert that the figure of the structm^e of 



