ICHTHYODORTTLITES. 
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. 
Tor 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 Cochliodontidae. 
Onchus, CtenacantJius, Anaclitacanthus, EunemacantJms, 
Homacanthus ~ Hoplonchus , v Acondylaccinthus, Aster op ty- 
chiusCosmacanthus, Bythiacanthus, Gly m mat acanthus, 
Thaumatacanthus, Chalazacanthus, Lispacanthus, Lepra- 
canthus , '’Nemacanthus , *(?) Psilacanthus. 
II. Slender elongated spines, bilaterally symmetrical, with the 
internal cavity only open at the base, and little or no 
smooth inserted portion. 
Gnathacantlius , 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. 
Machcer acanthus, Haplacantlius , Heteracanthus, Psam- 
mosteus , Stethacanthus, Physonemus, (?) Batacanthus, Sti- 
chacanthus , Oracanthus, Antacanthus, Gyracanthus, Agana- 
canthus. 
IY. 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. Hyboclus). 
Erxsmacanthus, Gampsacantlius, Lecracanthus, Dipria- 
canthus, Listracanthus, Byssacantlius, Oyrtacanthus, Eua- 
canthus, Harpacanthus, OstraccicantJius. 
Y. Dermal defences of doubtful position. • 
Edestus, Gynopodivs, Euctenius. 
