XXIV 
INTRODUCTJOX. 
it api)ears tliat tljis Permian fossil is a normal example of an 
amphistylic Elasraobrancli skull. One feature in the appendicular 
skeleton, hou’cvcr, amply justifies the recognition of the Ichthyotonii 
as a distinct order, if appeal may be made to the analogy of the 
accepted classification of the bony hyostylic fishes. The possession 
Fig. li. 
Restoration of Pleumcanthus gaudryi. — Coal- Measures, t'ommentry, France. 
[After Charles Brongniart.] 
of a pectoral archipterygium, of the type conceived by Gegenbaur, 
differentiates the Crossoptcrygii from the higher Ganoidei or Actino- 
pterygii ; and the same character separates the Ichthyotomi 
though perhaps less widely — from the Selaehii. The division of the 
pelvic girdle into two distinct halves is also a feature of significance ; 
and the primitive character of the axial skeleton is worthy of note. 
According to Prof. Cope’s latest proposition in regard to classifica- 
tion \ the presence of a double series of interspinous cartilages will 
also enter into the ordinal definition “ ; but the characters of the 
median fins themselves, though in many respects primitive, cannot 
be quoted at present as of more than family rank. 
Having thus eliminated the Ichthyotomi, and attempting no 
further division into suborders, on account of the slight character 
of available evidence, it remains to determine the possible classifi- 
cation of the Selaehii into minor groups. As is well known, most 
zoologists divide these roughly into Sharks and Pays, in accordance 
with the lateral or inferior situation of the gill-clefts ; Owen seems 
to overestimate a character of family value (the possession of dorsal 
fin-spines), and thus adds a third suborder; Gill adopts four 
divisions (“ suborders ”) of Sharks, based upon the characters of the 
' Auier. Xat. vol. xxi. (1887), p. 1015. 
» This feature, however, has not been noted in Chvndrmvhehjs ■ but the circum- 
sUne« is perhaps explained by the imperfect preservation of known specimens. 
