ISTRODUCTION. 
XIX 
not by the mesopterygium, as maintained by Huxley Balfour 
and Howes In the majority of Selachians, however, the ijectoral 
is completely shortened and so much modified that the homologies 
of the parts are diflBcult of determination ; and this stage appears to 
have been already reached even in a Lower Carhoniferous Shark *, 
which possessed a dentition indistinguishable from that of Cladodua. 
In the pectoral fin of the Lower Liassic Pakeospinax the type 
characteristic of the modern Shark is also evident, and there are a 
few robust dermal fin-rays. It is uncertain, however, whether any 
genus at this time, or of prior date, had heoome possessed of cartila- 
ginous pectoral fin-rays so enormously developed as to constitute a 
fin comparable to that of the existing Rays. The fossils named 
Artliropteni^ and Cyclartlirus (p. 150), from the Lias, are supposed 
to be most satisfactorily interpreted as being parts of such fins ; 
but the specimens are scarcely sufficient for profitable discussion. 
The Low'er Carboniferous teeth named Psammodus (p. 99) are also 
most nearly parallelled at the present day by those of fishes with 
an extremely depressed trunk and enormously developed pectorals 
without dermal rays ; but these fossils likewise afford no basis for 
reasonable speculation. 
Pelvic Arch and Fins. 
The pelvic fins in the earliest genua in which they are known 
(Pleuracanthus) exhibit the usual series of rays upon one side of 
the basipterygial axis ; and in the male there is a distinct appended 
clasper. The pelvis of Pleuracanthus is more singular than the 
pair of fins, inasmuch as it consists of two triangular cartilages, one 
on either side, only meeting and not united in the mesial line. 
The pelvic arch and fins in the extinct Jlesozoic genera are known 
in but few instances ; and only one case is worthy of special 
remark. In a Cretaceous member of the Trygonidse ( Ch/c7o6atw), 
the arch seems to be modified for the support of the metapterygium 
of the enormously developed pectoral fins. The “ prepubic ” process 
{p.ph., fig. A, p. xx) in this genus is much elongated and produced 
forwards; and a lateral process (i7.) on either side, apparently 
^ Proc. Zool. Soc. 1876, p. 55. 
= Comparative Embryology, vol. ii. (Reprint, 1885), p. 617. 
® Proc. Zool. Soc. 1887, p. 24. 
* Sphenacanihiis cosfe/latus (p, 242). 
b 2 
