IXTRODXrCTlOS. 
XXI 
fia in the anal region ; it is divided into two distinct members, 
closely following each other, and exhibiting a singular arrangement 
of the cartilages, most nearly parallelled, so far as known, in the 
first dorsal fin of the common existing Raja *. 
Shagreen and Dermal Defences. 
A noteworthy feature in the Palseontological history of the Elas- 
mobranchs is the relatively great development of the exoskeleton in 
the majority of the early genera. Pleuracantlms and Chondren- 
chelys, it is true, seem to have been almost, if not quite, destitute 
of shagreen; and the former, at least, possessed but a single spine. 
But the abundance of Ichthyodorulitcs, both paired and median, 
in the older rocks, testifies to the frequent armature of the body ; 
andilesoroic genera, like Ihjbodus and Acrodus, show how not only 
the dorsal fins, but also the sides of the head, were provided with 
formidable spines. 
iloreover, the early types of shagreen and spinous defences are 
remarkable for their degree of sculpturing. The small conical 
tubercles in the skin of Hgbodas and Acrodus (and the presumably 
equivalent Carboniferous fossils named Petrodus) exhibit a some- 
times elaborate stellate ornamentation ; and these bodies do not 
usually form so compact and continuous an investment as the small 
quadrate granules, characteristic of modern Sharks, which are 
apparently first met with in Janassa, of the Permian. 
Smooth dorsal fin-spines are also rare in Palmozoic rocks, being 
only known at present in the Carboniferous Pleuroplax and llelodus ; 
and the earliest smooth spines covered with ganoine are those of the 
E.h®tic and Liassic Pakeospinax. Except in Pleuroplax and Uelodus, 
the spines of Palaeozoic age are all more or loss sculptured upon the 
sides, or exhibit two series of powerful denticles. In the sculptured 
laterally-compressed forms of this period, the denticles are placed 
longitudinally in a scries upon each edge of the posterior face ; but 
in later times very few spines, except those of Chimaeroids, continue 
to exhibit such an arrangement, the denticles becoming still more 
prominent and approximating to form a double series along the 
middle of the posterior face. 
Paired spines doubtless referable to Elasmobranch or Chim»roid 
fishes are numerous in Carboniferous strata, and will be treated in 
the section upon Ichihyodoeulites *. Many are triangular, and 
’ See figure by Mivart, Trans. Zool. Soc. vol. x. pi. Ixxviii. fig. 7. 
^ Following the Holoeephali, in Part II. of this Catalogue. 
