INTRODUCTION. 
XIX 
not by the mesopterygium, as maintained by Huxley \ Balfour 2 , 
and Howes 3 . In the majority of Selachians, however, the pectoral 
is completely shortened and so much modified that the homologies 
of the parts are difficult of determination ; and this stage appears to 
have been already reached even in a Lower Carboniferous Shark A , 
which possessed a dentition indistinguishable from that of Olodoclus. 
In the pectoral fin of the Lower Liassic Pdlceospinax 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 become possessed of cartila¬ 
ginous pectoral fin-rays so enormously developed as to constitute a 
fin comparable to that of the existing Bays. The fossils named 
Arthrojpterus and Oyclarthrus (p. 156), 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 Lower Carboniferous teeth named Pscimmodus (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 ravs : but these fossils likewise afford no basis for 
■/ / 
reasonable speculation. 
Pelvic Arch and Fins. 
The pelvic fins in the earliest genus 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 Hesozoic genera are known 
in but few instances; and only one case is worthy of special 
remark. In a Cretaceous member of the Trygonidte ( Cyclobatis ), 
the arch seems to be modified for the support of the metapterygium 
of the enormously developed pectoral fins. The “ prepubic ” process 
( p.pb ., fig. A, p. xx) in this genus is much elongated and produced 
forwards; and a lateral process (il.) on either side, apparently 
1 Proc. Zool. Soc. 1876, p. 55. 
2 Comparative Embryology, voh ii. (Reprint, 1885), p. 617. 
3 Proc. Zool. Soc. 1887, p. 24. 
4 Sphcn acanthus costellatus (p. 242). 
h 2 
