246 NATURAL SCIENCE. ahril. 



of Palaeozoic sharks in a condition that shall render the study of 

 their structure possible. At the present time, enough at least 

 of the fossil forms have come to light to convince so critical a 

 palaeontologist as Mr. A. S. Woodward that " if the earHest true fish 

 could be found, it would almost certainly fall within the sub-class 

 Elasmobranchii."' 



Three early sharks have, within the past few years, been some- 

 v/hat definitely described. The earliest of these, Cladoselache [Cladodns), 

 is, however, as late as the Lower Carboniferous. Next to this in 

 antiquity is Chondrenchelys, from the Carboniferous; and the latest, best 

 known in the Permian, is Plenmcanthus (and X en acanthus). Previous 

 to the description of these early types, the Acanthodians, members 

 of a highly specialised group of the Elasmobranchii, had alone been 

 structurally known. ^ 



The features of these early forms may now be summarised with 

 a view of determining what evidence they afford as to the ancient 

 descent of the sharks. 



Pleuracanihus, although the latest of these types, appears, never- 

 theless, as the structural studies of Cope, Fritsch, Brongniart, and 

 Jaekel have shown, to have been decidedly unlike recent sharks in 

 many of its features. It may, perhaps, represent the advancing stem 

 of the Elasmobranch group, which may early have given rise to 

 Teleostomi or to the Lung-fishes. At all events, Pletiracanthus gives us 

 but little clue to the structures of the primitive sharks. We find, for 

 example, that it had nearly lost its body-armouring of shagreen^ : that it 

 had evolved archipterygial fins, and a diphycercal tail ; that it had pro- 

 duced a spine, elaborately specialised, and remarkable in its anterior 

 position. Add to these characters the fact that no more than five 

 gill arches were present, and that the upper element of the man- 

 dibular arch had become differentiated to a degree which rendered 

 the skull amphistyUc^ Moreover, the most important primitive 



1 Natural Science, vol. VI., p. 38. Jan., 1895. 



2 Add to these Symmoriuni, a Permian form recently described by Professor Cope. 



'^That it had a body-armour of shagreen seems evident from a study of Dr. 

 Brongniart's material in Paris, for a patch of typical denticles occurs in the male, 

 near the base of the claspers. Cf. also Jaekel, SB. Gesell. nat. Freunde Berlin, no. 4, 

 p. 77; 1895. 



*To these highly-evolved features of Pleiiracantlius might perhaps be added (i) 

 the unsegmented character of the main body of the gill-bars (as in Lung-fishes), 

 (2) the presence of teeth in the pharynx, (3) the greatly calcified condition of the 

 skeleton — a feature of more or less value in connection with other advancing or 

 specialised conditions. Two other primitive features are noted by Jaekel {op. cit., 

 p. 72), (i) the paired character of the hypobranchial, and (2) the segmented condi- 

 tion of the shoulder girdle. The present writer, while inclined to accept the first of 

 these conditions as primitive, must note, nevertheless, that the copulae of the other 

 arches are so widely differentiated that it is clearly possible that the paired hypo- 

 branchials might as well represent adaptive structures. With regard to the second 

 point, having, through the kindness of Dr. Jaekel, examined the Berlin material, 

 the present writer cannot but believe that the segmented nature of the shoulder 

 girdle is due entirely to accidental fracture. 



