﻿ICHTHTODOHULITES. 93 



probably referable, for the most part, to one or the other of the sub- 

 classes just mentioned, but cannot yet be precisely determined. 



The various " genera " already recognized may be briefly defined 

 and discussed ; but, although it is convenient to adopt provisional 

 specific names for such fossils, future discoveries may soon lead to a 

 more precise systematic allocation of most of the forms, and it will 

 thus suffice merely to refer to the published diagnoses, without 

 repeating them. 



For convenience of reference, it is proposed to arrange the Ich- 

 thyodorulites in five groups, as follows : — 



I. Slender elongated spines, bilaterally symmetrical, the inserted 



portion smooth and usually sharply separated from the 

 ornamented exserted portion ; internal cavity open poste- 

 riorly towards the base. Resembling the dorsal fin-spines 

 of the Cestraciontidae, and probably for the most part 

 referable to that family and to the Cochliodontidse. 



Onchus, Ctenacanthus, Anaclitacanthus, Eunemacanthus, 

 Homacanthus, Hoplonchus, Acondylacanihus, Asteropty- 

 chius, Cosmacanthus, Bythiacanthus, Glymmatacanthus, 

 Thaumatacanthus, Chalazacanthus, Lispacanthus, Lepra- 

 canthus, Nemacanihus, (?) Psilacanihus. 



II. Slender elongated spines, bilaterally symmetrical, with the 



internal cavity only open at the base, and little or no 

 smooth inserted portion. 



Gnathacanthus, Apateacanthus, Pristacanthus, Codorhyn- 

 chus. 



III. Paired spines, of which some may have been placed in front 



of fins, but of which many are broad, with insignificant 

 base of insertion, and must have been arranged as inde- 

 pendent dermal armour. 



Machceracanthus, Haplacanthus, Heter acanthus, Psam- 

 mosteus, Stethacanthus, Physonemus, (?) Batacanthus. Sti- 

 chacanthus, Oracanthus, Antacanthus, Gyracanthus, Agana- 

 canihus. 



IV. Spines probably not placed in advance of fins, but most 



nearly resembling the head-spines of the male Chimaeroids 

 and some Mesozoic Cestraciont Sharks (e. g. Hybodus). 



Erismacanihus, Gampsaeanthus, Leer acanthus, Dipria- 

 canthus, Listr acanthus, Byssacanthus, Cyrtacanthus, Eua- 

 canihus, Harpacanthus, Ostracacanthus. 

 Y. Dermal defences of doubtful position. 



Edestus, Cynopodius, Euctenius. 



