HARDW1CKKS SCIENCE-GOSSIP. 



i55 



some scattered teeth from the Chalk, under the name 

 of Ccstrario7i canaliculatus, because they seemed to 

 differ but little from those of the recent Cestracion, 

 except in their smaller size and the possession of a 

 minute channel passing obliquely through the root of 

 each. Three years after this, in 1S53, the same 

 ichthyologist announced the discovery of a specimen 

 proving the teeth and spines to belong to one fish ; 

 and in 1S72 Sir Philip, also, published detailed 

 descriptions of all the more important specimens 

 then available, and proposed the generic name by 

 which this Selachian is now known.* Fig. 106, A, B, 

 are drawings (half nat. size) of the first and 

 second dorsal fin-spines, which are only marked by 

 lines of growth and do not appear to have been very 

 deeply implanted in the soft parts ; and figs. 103, 104 

 represent an anterior and posterior tooth, the former 

 quite prehensile, and the latter adapted for crushing, 

 as is the case in the front and back teeth of 

 Cestracion. D. canaliculars is the only species 

 of the genus at present recognised, and its remains 

 occur chiefly in the Chalk, although other English 

 Cretaceous deposits have yielded a few fragments. 



Rhinid^;. 



Our object in this series of articles being to 

 dwell chiefly upon those Selachian fossils that 

 most commonly come under the notice of English 

 collectors, and to summarise the results of the latest 

 researches relating to such, a passing notice will 

 suffice for the small, but interesting family of 

 " Angel-fishes " and " Monk-fishes." None of their 

 remains are known to occur in British strata, and the 

 Lithographic Stone (U. Oolite) of Bavaria and 

 France appears to be the only Continental deposit 

 yielding examples of importance. These have been 

 referred to the living Rhina (= Sqnatina) and the 

 doubtfully distinct genus Thawnas : though the gill- 

 openings are lateral, the general form of the body is 

 much like that of the Rays, and there are no dorsal 

 spines. 



Pleuracanthid.e (Xenacanthid.e). 



This is an extinct family, of which much yet 

 remains to be learned. It comprises the various 

 forms that have been described at different times 

 under the generic names of Pleuracanthus, Diplodus, 

 Orthacauthus, Xenacanthus. and Triodus, and which 

 it is now almost universally agreed to unite under the 

 first (the earliest) of these terms. Triodus is un- 

 doubtedly identical with the previously-described 

 Xenacanthus, and there is no doubt, likewise, that 

 this is the same as Pleuracanthus. The chief dis- 

 puted point is, whether Pleuracanthus and Ortha- 

 canthus really differ generically, or merely specifically, 



* Mem. Geol. Surv., Dec. XIII. 



and the most recent contribution* to the subject, by 

 Mr. J. W. Davis, of Halifax, seems to show that the 

 latter is most probably the case. 



The ordinary fossil remains of this family met 

 with in Britain, are confined to Carboniferous strata, 

 and present themselves in the form of detached spines 

 (called Pleuracanthus and Orthaeanthus) and teeth 

 (known as Diplodus), but the Continental specimens, 

 to which we shall shortly refer, are much more 

 complete and occur chiefly in the Lower Permian. 

 The spines are long, usually straight, and tapering 

 to a point, with a smooth or finely striated surface, 

 upon some part of which are arranged two longitudinal 

 rows of denticles ; they much resemble the spines of 

 recent Rays in external shape, but differ from those 

 of such as Trygon and Myliobatis in not being solid, 

 but possessing a hollow cavity which opens at the base. 

 Fig. 107 represents a typical example of the Pleuracan- 

 thus spine, half the natural size, and the diagrammatic 

 transverse sections, figs. 108, 109, show the difference 

 between this and the form originally termed Ortha- 

 eanthus ; the latter, it will be observed, is much 

 more cylindrical than the former, and the rows of 

 denticles are placed close together along the back, 

 instead of far apart along the sides, but in the paper 

 already mentioned, numerous intermediate forms are 

 described, which demonstrate that these are only the 

 two extremes of a nearly continuous series. 



The little bodies known as Diplodus (fig. no) consist 

 of a thick bony base, upon which are fixed two 

 comparatively large diverging denticles, with a 

 smaller denticle and a little flat-topped or rounded 

 boss rising between. They occur not unfrequently 

 at many Coal Measure localities, and considerable 

 numbers are sometimes met with in association. 

 Agassiz originally described them as teeth, and this 

 seems to be the view now generally accepted, but 

 some palaeontologists have expressed the opinion 

 that they are simply dermal tubercles analogous to the 

 prickles of the " Thornback " and other recent Rays.f 



The Permian specimens of Pleuracanthus {Xena- 

 canthus) found in Germany elucidate many important 

 details in the anatomy of the interesting Selachians 

 whose fragmentary remains have just been noticed. 

 Some examples, in fact, exhibit nearly all the hard 

 parts of the fish in their proper relative positions. 

 The body is slightly flattened, and the general shape 

 recalls that of Rhina ; there are numerous teeth, of 

 the Diplodus type, % in the jaws, and the large 



* Quart. Journ. Geol. Soc. London, vol. xxxvi. (1880), 

 pp. 331-336. References to previous literature are here given. 

 t "Ann. and Mag. Nat. Hist." [4] vol. i., 1868, p. 371. _ 

 { We may here note that this type of tooth is not exclusively 

 confined to Pleuracanthics, having been found in association 

 with at least one other spine in the Lower Carboniferous, (T. 

 Stock, "Nature," vol. xxvii. 1882, p. 22). Further, recent 

 numbers of the American Scientific Journals contain notices 

 of a new Shark, named Chlamydoselachus from the Japanese 

 seas, of which the dentition is exceedingly similar ; in fact, 

 Professor Cope has ventured to refer the latter to the Palaeozoic 

 genus, but the figures show the fish to be very different in form 

 and indicate the absence of a spine. 



