Revieived by Dr, Christian Lutken. 379 



not being a true sucking-disc, as in the CyclopUri, but a male copu- 

 lative organ. That the XenacantJii fed upon their weaker contem- 

 poraries the Acanthodes, is evidenced by the discovery of spines and 

 other remains of this genus within the body of the former. As to its 

 geological position, it is characteristic of the " Rothliegende " of the 

 Permian system, and our author thinks that the strata in England 

 and North America containing teeth of Diplodus and spines of Pleura- 

 canthus, ought, perhaps, on closer examination, also to be referred to 

 the same system, and not to the Carboniferous. 



In summing up the characters of this peculiar palaeichthyic type 

 with the view of determining its zoological affinities and systematic 

 position, we should, with Dr Kner, distinguish between those which 

 it has in common with modern Chondropterous (Placoid) fishes; for 

 instance, the mosaic or tessellated covering of the skeleton, the absence 

 of opercular bones, the position of the shoulder-girdle, the single un- 

 divided suspensorium of the jaw, the articulation of the upper jaw 

 on the lower, the dermal covering, etc., all which of course would 

 indicate the necessity of an arrangement, by which the Pleuracanthi 

 would be placed among the Chondropteri, in the vicinity of the sharks. 

 On the other hand, there are characters which contradict this position, 

 and which remind us rather of the true osseous fishes ; for instance, 

 the dorsal fin-rays, the double row of interspinal bones, the toothed 

 inter-maxillary, palatine, pharyngeal, and branchial arches, the tri- 

 partite shoulder-girdle and the branchiostegal rays. Professor Kner 

 thinks these latter arguments so heavy that the balance inclines 

 towards the Teleostei, especially the Siluri, rather than towards the 

 Chondropteri, but that Xenacanthus was nevertheless an intermediate 

 form partaking of the characters of each. We must confess, how- 

 ever, that the analogies with the " Sheet "-fishes (Flat-fishes) 

 pointed out by the author ; as for instance, the lengthened body, the 

 broad terminal mouth, the shagreen covering of the head, recalling 

 the '' helm " of Bagrus, the straight and toothed dorsal spine (com- 

 pressed, however, and serrated before and behind in the •' Sheet "- 

 fishes, depressed and serrated laterally in Xenacanthus, as pointed out 

 by the author himself, the long many-rayed dorsal fin, continued to 

 or around the long attenuated tail ; the appendages of the ventral fins 

 in the males (recalling, however, much more those of the sharks,) 

 appear to us to be only vague analogies rather than evidences of 

 afl&nity. Even if Xenacanthus difi'ered from the Chondropteri in all 

 the above-named parts of its organisation, and for some of them we 

 must entirely rely on the sagacity and accuracy of the author, (his 

 figures not always giving convincing proof or the means of checking 

 his evidence,) while others, for instance the branchiostegal rays may 

 be found in Chimgeroids. I should think that Xenacanthus differed 

 after all not much more from the typical Chondropteri (viz., the 

 Plagiostomi (Skates and Sharks), though in a diff'erent manner and in 

 another direction, than does e. g. the Chimara, which is nevertheless 

 commonly and duly regarded as a true Placoid, closely related to 

 Sharks. Might we, perhaps, not come nearest to the truth by estab- 

 lishing in the order of Chondropteri a peculiar tribe or sub-order for 



