44 
DAVIS : FOSSIL TISH EEMAINS. 
Dimyleus Woodi, Agass. 
Petalodus accuminatus, Agass. ... 
Polyrhizodus, sp. ? 
Petalopsodus tripartitis, Davis. 
Ctenopeta,lus serratus, Agass, ... 
Petalorhynclius psittacinus, Agass. 
Pristodus falcatus, Agass. 
Megaliclithys, sp. ? 
Leyburn. 
„ Settle, Richmond. 
?> 
„ Richmond. 
5? 
The whole of the fish remains enumerated above are comprised 
in the sub-class, PalcEichthytes as defined by Dr. Gunther. The 
sub-class includes two orders, the Chondropterygii and the 
Ganoidei. The latter is equivalent to Prof. Huxley's Ganoidei and 
Dipnoi, whilst the former comprises the Elasmobranchii of Prof. 
Huxley's classification and includes the sharks, rays, and chimeras. 
The order Chondropterygii is divided into two sub-orders: 
Plagiostomata, comprising- the sharks and rays, and the Holocephala, 
the chimeras. The Elasmobranch fislies of the Mountain Limestone 
are all included in the Sub-order Plagiostomata. The i-emains of 
these fishes which have been preserved are always in a disinteg- 
rated condition and consist of detached teeth or spines. A slight 
consideration of the anatomy of an existing- shark will at once 
explain the reasons why this is so. The framework of a shark is 
cartilaginous ; the mandibles, skull, supports for the fins, &c. are 
each unossified, though well developed ; and the vertebrae are 
also in many cases cartilaginous though various modifications occur. 
In some there is a slight ring of bone imbedded in the cartilage, 
in others the whole of the vertebra is ossified. The vertebrae of the 
plagiostomous fishes whose remains are found in the Carboniferous 
Limestone appear to have been entirely cartilaginous, for hitherto 
no trace has been found of their preservation. The only parts of 
the fish which have been preserved are the teeth, the spines which 
occupied positions in front of the dorsal or pectoral fins, and 
occasionally patches of the dermal covering like shagreen. The 
organs on which, since the researches of Johannes Mtiller, modern 
classification has been based — the heart, the spiral intestine and the 
