

84 



Proselachii Ichthyotomi. 



Table-case. 

 No. C. 



bones, and shagreen granules : the teeth probably bear another 

 name. Acondylacanthus resembles the spine of a Bay ; and 

 Erismacanthus and Lispacanthus are very suggestive of the 

 rostral spine of male Chimeeroids. 



These ichthyodorulites show that the earliest Elasmo- 

 branchs differed very much from the existing members of the 

 sub-class. The fact is further emphasised by the discovery of 

 several Elasmobranch skeletons of Devonian, Carboniferous, 

 and Permian age, which show that at least three extinct orders 

 are represented. 



ORDER I. Proselachii. 



The first of these orders is that of the PROSELACHII, 

 represented only by Cladoselache, from the Upper Devonian 

 of Ohio, U.S.A. This is a notochordal shark without spines or 

 armour, except a ring of plates round the eye ; its paired fins 

 are mere balancers supported by parallel rods of cartilage, and 

 the teeth are identical with the detached Palaeozoic teeth 

 provisionally named Cladodus. Only fragments are exhibited. 



Wall-case, 

 No. 3. 



Table-cs se, 

 No. C. 



ORDER II. Ichthyotonii. 



The second order, that of the ICHTHYOTOMI, is known by 

 numerous tolerably complete skeletons from the Carboniferous 

 and Lower Permian. Fine examples of the typical genus 

 Pleur acanthus (Fig. 112) are exhibited from the Lower Permian 

 of Rhenish Prussia, Silesia, and Bohemia. The isolated spines 

 of Pleur acanthus and the teeth known as Diplodus from the 



FIG. 112. Pleuracanthus Gaudryi, Broiign. (restoration by 0. Brongniart) ; Coal-measures, 

 Commentry, France. 



British Carboniferous, especially Coal-measures, are also placed 

 here. Like the Proselachii, these sharks are notochordal ; but 

 they differ in having paddle-shaped paired fins, supported by a 

 more or less branching arrangement of cartilages. The median 

 fins are very extensive, and there is usually a median dorsal 

 spine fixed at the back of the head. There is reason to believe 



