xviii 
INTRODUCTION. 
bo detected. Another Cestraciont genus of Cretaceous ago {Syne- 
chodus), which can scarcely be distinguished from Palceospinax in 
the characters of its dentition and external dermal structures, 
possesses fully-formed vertebrae of the asterospondylic type. 
Truly asterospondylic vertebrce, indeed, are already mot with in 
the Ceslracion falcifer (p. 332) of the Lithographic Stone ; and the 
representatives of Sqvatina and Eldnohatus of the same age furnish 
equally typical examples of well-formed toctospondylic vertebr®. 
In the Jurassic species of Sqnadna, however. Dr. Hasse ‘ has 
pointed out that the number of peri])heral calcified rings in the 
vertebra; 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 cither side, probably separated in the median line. It would 
bo 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 Tcetospondyli 
to form a complete girdle ; but evidence upon all these points is at 
present wanting. 
Each new discovery of the most primitive types of Elasraobranchs 
seems to render the conclusion more certain, that the earliest stage 
of the pectoral fin was that named the “ archipterygium ” by 
Gegenbaur". As pointed out by Goldfuss and Kner, and more 
recently by Anton Fritsch and C. Brongniart, this appendage in the 
Palaeozoic Pletiracanfhns exhibits a long segmented axis, fringed on 
cither side with cartilaginous rays ; and a nearly similar arrange- 
ment has lately been discovered bj^ Traquair in Cladodvs (p. 16), 
though in this genus the fin may have possessed ra}’s only upon one 
bide 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 ^ and, if the known examples of the pectoral fin 
of Pleuracanthus sufSce for philosophical discussion, the central 
axis is formed by the metapterygium, as Gegenbaur supposed, and 
' Natiirl. Syst. Elasmobr., Besond. Theil, p. 132. 
’ 0. Gegenbaur, “ Ueber das Archipterygium,” Jena. Zeitschr. vol. vii. (1873) 
pp. 131-141, pi. X. 
’ C. Gegenbaur, ihid. 
