INTRODUCTION. 
XXL 
fin 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 l . 
Shagreen and Dermal Defences. 
A noteworthy feature in the Palaeontological history of the Elas- 
mobranchs is the relatively great development of the exoskeleton in 
the majority of the early genera. PIeuracanthus and Chonclren- 
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 Ichthyodorulites, both paired and median, 
in the older rocks, testifies to the frequent armature of the body ; 
and Mesozoic genera, like Hybodus and Acrodus , show how not only 
the dorsal fins, but also the sides of the head, were provided with 
formidable spines. 
Moreover, the early types of shagreen and spinous defences are 
remarkable for their degree of sculpturing. The small conical 
tubercles in the skin of Hybodus 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 Janassci, of the Permian. 
Smooth dorsal fin-spines are also rare in Palaeozoic rocks, being 
only known at present in the Carboniferous Pleuroplax and Heloclus; 
and the earliest smooth spines covered with ganoine are those of the 
Mhaetic and Liassic Palceospinax. Except in Pleuroplax and Heloclus , 
the spines of Palaeozoic age are all more or less 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 series 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 Chimaeroid 
fishes are numerous in Carboniferous strata, and will be treated in 
the section upon Ichthyodorulites 2 . Many are triangular, and 
1 See figure by Mivart, Trans. Zool. Soc. vol. x. pi. Ixxviii. fig. 7- 
2 Following the Holocephali, in Part II. of this Catalogue. 
