PISCES. 
63 
Sub-class I.—Elasmobranchii. 
Most of the fossil remains of sharks, dog-fishes, skates, 
and their extinct representatives, are very fragmentary, on 
account of the imperfect hardening of the internal skeleton 
by lime. In many cases only scattered teeth, spines, and 
hard skin-tubercles or “ shagreen ” remain. The detached 
fin-spines are rarely sufficient to indicate the nature of the 
fishes to which they originally belonged, and they are only 
referred to the Elasmobranchii because they have the same 
microscopic structure as the spines of modern sharks and 
skates. These fossils are named Ichthyodorulites (“ fish- 
spine-stones ”), and are arranged in the small Table-case F in 
the middle of the Gallery. Among them may be specially 
noticed the small ribbed spines of Onchus from the Lower Old 
Red Sandstone; Oracanthus from the Carboniferous Lime¬ 
stone ; and Listracanthus from the Coal Measures. One of 
the largest known Ichthyodorulites is Oracanthus pustulosus, 
26 inches in length, from the Carboniferous Limestone of 
Bristol, in Wall-case 2. 
Order I.— ACANTHODII. 
The oldest Elasmobranchs are the small Acanthodian 
fishes, ranging from the Upper Silurian to the Upper 
Permian. They are completely covered with regularly 
arranged shagreen, and they often exhibit a ring of plates 
Fig. 60.— Restoration of Acanthodes wardi, froml the Goal Measures of 
Staffordshire; about one-third nat. size. (Table-case 1.) 
round the eye. When teeth are present these are firmly 
fixed to the edge of the jaws. The tail is heterocercal, and 
each of the other fins is armoured with a spine in front. 
The Upper Silurian and Lower Devonian Acanthodians 
Table-case 
F. 
Wall-case 
1 . 
Table-case 
1 . 
