xviii 
INTRODUCTION. 
be detected. Another Cestraciont genus of Cretaceous age (Syne- 
chod-us), which can scarcely be distinguished from Paloeospinax in 
the characters of its dentition and external dermal structures, 
possesses fully-formed vertebrae of the asterospondylic type. 
Truly asterospondylic vertebrae, indeed, are already met with in 
the Cestrcicion falcifer (p. 332) of the Lithographic Stone ; and the 
representatives of Squatina and JRhinobatus of the same age furnish 
equally typical examples of well-formed tectospondylic vertebrae. 
In the Jurassic species of Squatina, however, Dr. Hasse 1 has 
pointed out that the number of peripheral calcified rings in the 
vertebrae is less than in the later species of the same genus. 
Pectoral Arch and Fins. 
The remains of the pectoral arch in the earliest known Elasmo- 
branchs indicate that it consisted of a pair of arched cartilages, one 
upon either side, probably separated in the median line. It would 
be interesting to know at what period, and in what form, the sepa¬ 
ration of the supra-scapular cartilage in the Rays first occurred, and 
how early the two lateral elements united in any of the Tectospondyli 
to form a complete girdle ; but evidence upon all these points is at 
present wanting. 
Each new discovery of the most primitive types of Elasmobranchs 
seems to render the conclusion more certain, that the earliest stage 
of the pectoral fin was that named the “ archipterygium ” by 
Gegenbaur 2 . As pointed out by Goldfuss and Xner, and more 
recently by Anton Eritsch and C. Brongniart, this appendage in the 
Palaeozoic Pleuracanthns exhibits a long segmented axis, fringed on 
either side with cartilaginous rays; and a nearly similar arrange¬ 
ment has lately been discovered by Traquair in Clcidodus (p. 16), 
though in this genus the fin may have possessed rays only upon one 
side of the longitudinal axis. There is still some slight approach 
to such an archipterygial type in the pectoral fin even of a few 
living Selachians 3 , and, if the known examples of the pectoral fin 
of Pleuracanthns suffice for philosophical discussion, the central 
axis is formed by the metapterygium, as Gegenbaur supposed, and 
1 Katiirl. Syst. Elasmobr., Beson/i. Theil, p. 132. 
2 C. Gegenbaur, “Ueberdas Archipterygium,” Jena. Zeitschr. vol. vii. (1873) 
pp. 131-141, pi, x. 
3 C. Gegenbaur, ibid. 
