436 



NATURAL SCIENCE. 



June, 



The Acanthodian fishes, discovered in Palaeozoic rocks from the 

 Lower Devonian upwards, have always presented difficulties to 

 systematists ; but it is now generally agreed that they are a peculiar 

 order of sharks. As to the precise significance of several parts of 

 their skeleton, however, there is still much disagreement, and Dr. 

 Fritsch himself wisely refrains from very definite expressions of 

 opinion. 



The general aspect of the type-genus Acanthodes is shown in Dr. 

 Fritsch's outline figure reproduced below. The cartilages of the head 

 are calcified with granulations, as in sharks ; and there are splints 

 covering the jaws, but neither these nor any other skeletal parts have 

 the structure of bone. The roof of the skull is covered at least in 

 part with small dermal plates, and there is a ring of plates round the 

 eye. The gill-arches are so much crowded that Dr. Fritsch thinks 

 the gill-slits must have been covered by a flap of skin, as in the existing 

 Chimaeroids. The notochord must have been persistent, and 

 there are rarely traces of the neural and haemal arches above and 

 below the vacant space it once occupied. The tail is heterocercal, 



Fig. I. — Outline restoration of Acanthodes hronni; Permian, Rhenish Prussia. 



{After Fritsch.) 



the upper lobe being much produced ; and in front of all the fins, 

 except the caudal, there is a strong spine. The body is covered with 

 little quadrangular scales, consisting of dentine enamelled at the 

 surface ; and the " lateral line " does not pierce these scales, but runs 

 between two series. 



Among the Permian Acanthodians Dr. Fritsch recognises three 

 genera, one of which is destitute of pelvic fins. The large size of 

 the paired fins, however, is one of the chief characteristics of 

 Acanthodes itse.\i\ and, as pointed out by Professor Cope, there is 

 more evidence of a continuous pair of lateral fin-folds in these Acan- 

 thodians than in any other fishes. Some of the Devonian genera 

 {e.g., Climatius), exhibit between the pectoral and pelvic fins a close 

 and regular series of paired spines, in every respect identical with 

 those supporting the fins themselves ; and it is quite possible that 

 these intermediate spines may also have been connected with fin- 

 membrane. 



It is strange to find a group of fishes like the Acanthodians 

 among the sharks — fishes which at the present day never exhibit 

 structures of the nature of membrane-bones, and never have the 



