XX 
INTRODUCTION. 
homologous with the “ iliac/’ is not only of great length, but has 
also sharply reflected extremities, which seem to have been originally 
Fig. A. 
Pelvic cartilage of Cyclobatis oligodactylus. —Senonian, Mount Lebanon, bp., 
basal cartilage of pelvic fin ; il., iliac process; pb., pubic cartilage; p.pb., 
prepubic process. [From Proc. Zool. Soc. 1888, p. 128.] 
in direct connection with the distal end of the pectoral meta- 
pterygium. 
Median Fins. 
A Lower Carboniferous species of JSphenacanthus (p. 242) presents 
as highly specialized an arrangement of the median fins as most 
modern Sharks : but in the primitive Chondrenchelys of the same 
age, and in the late Palaeozoic PIearacanthus, a lower stage of 
development persists. In Chondrenchelys there was evidently a 
long undifferentiated median fin, with at least one series of slender 
supporting cartilages above the neural spines. In Pleuracanthus 
(according to C. Brongniart) a long dorsal fin is separated from the 
diphycereal caudal; and there is also said to be a small separate 
“ cephalic ” fin supported by the barbed spine—a feature of which 
the spine itself affords no evidence. As pointed out by P. Kner l , 
the series of interspinous cartilages supporting the dorsal fin is 
double ; and C. Brongniart shows these elements to be twice as 
numerous as the neural arches, each of the neural spines distally 
bifurcating to support them (see fig. B, p. xxiv). Very striking in 
Pleur acanthus, on the other hand, is the specialization of the median 
1 Sitzungsb. math.-naturw. Cl. k. Akaci. Wise. Wien, vol. lv. (1867), pis. i., 
ii., x. 
