2"]: 



HARDWICKE'S SCIENCE-GOSSIP. 



and as it is the custom to restrict the term to those 

 spines whose precise relationships are indeterminable, 

 in consequence of their never having been found in 

 association with definite fragments of the rest of the 

 fish-structures, to which they originally belonged, 

 the number of forms included in this category has 

 been considerably reduced by the progress of research. 

 Among those removed, for example, are the Devonian 

 Par exits (of which an outline is given in fig. 185), the 

 Carboniferous Ctinacanthus and Pkuracanthus, alluded 

 to in previous articles, the Jurassic Lcptacantlius, now 

 known to belong to a Chimseroid (Ischyodus) and 

 many others. But there are still a large number 

 that can be referred to no very definite place, and 

 though they may not all be truly Selachian — as is 

 quite probable, and has proved to be the case with 

 Parexus and Leptacantkus just mentioned — it will be 

 convenient to group the whole together until our 

 information becomes more complete. Some forms 

 appear to have been placed in front of dorsal fins (as 

 shown in the drawing oiSpinax, Vol. XX. p. 172, or 

 in fig. 185, a) ; others were almost certainly pectoral 

 or ventral fin-spines and perhaps situated like those 

 of the curious Acanthodian fishes (fig. 185, b) ; and 

 others, again, may correspond to the little triangular 

 dermal scutes (fig. 185, c) that are also to be found 

 in pairs in the same primitive tribe. Nearly all are 

 ornamented on the external surface with variously 

 disposed ridges and tubercles, often enamelled, and 

 the portion embedded in the tissues of the body— the 

 extent of which varies considerably in the different 

 forms — is usually smooth or finely striated.. More- 

 over, all agree in being destitute of any articular 

 facettes at the lower end, thus indicating the cartila- 

 ginous state of the skeleton with which they were 

 once connected ; and the presence of an internal 

 cavity, opening lengthwise behind and below, or 

 simply in a hole at the base, is also evidently a 

 constant character. 



The earliest of these dermal weapons hitherto 

 described is the little Onchus (fig. 186) from rocks of 

 Upper Silurian age. W ith the exception of Scaphaspis 

 Imlcnsis, from the Lower Ludlow of Leintwardine 

 (Shropshire), and the doubtfully piscine conodonts, 

 it is the oldest evidence of the existence of vertebrate 

 life yet known. In Britain, it occurs in the celebrated 

 Ludlow bone-bed, and in the passage beds between 

 the Silurian and Old Red Sandstone strata of the 

 same area, and quite lately, spines of a similar type 

 have been recorded* as occurring in beds of a slightly 

 earlier date in America ; but the remains originally 

 ascribed to this genus from strata of the carboniferous 

 period appear, from more recent discoveries, to 

 belong rather to such forms as Physoncmits and 

 Ctenacanthus. The external surface is characterised 

 by thick and smooth longitudinal ridges, and the 



* E. W. Claypole, Quart. Journ. Geol. Soc, xli. (1885), 

 p. 61. 



most distinctive feature of the spine is the absence of 

 posterior denticles. 



The Old Red Sandstone and Devonian — at least 

 in this country — yield no ichthyodorulites of im- 

 portance, but almost all divisions of the Carboniferous 

 are replete with examples of the greatest interest. 

 The largest forms hitherto discovered are met with 

 in the Carboniferous Limestone, Phoderacanthus 

 grandis* from Bristol, probably attaining a length of 

 no less than three feet. Among others, we may 

 especially refer to those known under the names 

 of Gyracanthus, Orthacanlhus, Acondylacanthus, 

 Lcpracanthus, and Erismacanthus. 



Gyracanthus is a genus first established by Agassiz, 

 and more completely elucidated since by the researches 

 of Hancock, Atthey, and Traquair, upon a much 

 larger series of specimens : it ranges throughout the 

 whole of the Carboniferous strata, though not yet 

 known to extend either above or below, and is so- 

 called in allusion to the peculiar appearance of the 

 spine produced by the arrangement of the ornamenta- 

 tion (fig. 187). The ichthyodorulite is characterised by 

 its very slightly compressed form, — almost round in 

 transverse section, — by an extensive internal cavity, 

 and a long base of insertion ; some examples are of 

 considerable size, attaining a length of sixteen or 

 eighteen inches, and almost all that are referable to 

 adult fishes exhibit a long worn surface at the tip 

 (fig. 187, a), evidently due to constant friction with 

 the bed or sides of the water in which their original 

 possessors lived. It is further noticeable that — in- 

 stead of being symmetrical — all these spines are 

 distinctly "lefts" and "rights," — a fact suggesting 

 that they occurred in pairs, and taken in conjunction 

 with the wearing of the tips, doubtless indicating 

 that they were placed in front of pectoral or ventral 

 fins. Messrs. Hancock and Attheyf believed, also, 

 that they had discovered a few symmetrical spines of 

 the same type, and hence regarded the latter as 

 dorsal ; but Dr. Traquair^ has more recently shown 

 that there is no unquestionable basis for such a con- 

 clusion, and considers it to have been founded upon 

 imperfect materials. Associated with the ichthyo- 

 dorulites, there often occur patches of small dermal 

 tubercles, and occasionally also curious triangular 

 bodies, once looked upon as the carpal cartilages of 

 Gyracanthus. These are hollow and open at the 

 base, with a roughened surface destitute of any 

 ornamentation, and, from a study of their micro- 

 scopical structure, Dr. Traquair has determined that 

 they are truly dermal appendages. The number ol 

 species already described from British rocks is about 

 seven, and the best known appear to be the 

 G. tubcrculatus and G. for/uosus, of which the coal 



* J. \V. Davis, Trans. Roy. Dublin Soc, Ser. 2. Vol. I. 

 (1883), p. 534, pi. lxv. Since description, the original specimen 

 of this hue ichthyodorulite has been presented by Earl Ducie 

 to the British Museum, where it is now to be seen. 



f Ann. and Mag. Nat. Hist., [4] I., 1868, p. 368. 



J Loc. cit., [5] XIII., 18S4, pp. 37-48. 



