600 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [June 20, 



to the margins of the scale, and about their own breadth apart. The 

 frequent interruptions of the concentric ridges gives the appearance 

 of coarse tuberculation, which is increased by the roughened condi- 

 tion of the summits of the isolated portions. The intervening inter- 

 spaces frequently show fine radial striae, which may, however, be 

 those of the deeper, exposed by erosion of the upper layers*. 



To the definition of the genus which Owen gave from the form of 

 the teeth only, M'Coy, in 1848 (Ann. ISTat. Hist,), added the cha- 

 racters of the scales, which he described as " thin, rotundo- quadrate, 

 with smooth under surface, and subcentral boss." If by '' thin," in 

 another place "rather thin," any marked difierence is implied be- 

 tween HolojptycTiius and Eliizoclus, the examples of the latter on 

 which the present article is founded do not confirm the implication. 

 Thin scales there are of the form stated by M'Coy ; but they never at- 

 tain the dimensions of the larger number of Rhizodus-^GdleB. They 

 are those of Rliizodojpsis, whose superficial ornament is, further, very 

 different. Portions of scales, one or more laminae of scales, are of not 

 uncommon occurrence ; and these, perhaps, have suggested the above 

 generic character. But the probability that M'Coy has mixed up 

 two genera, not then recognized as distinct, is increased by the or- 

 nament ascribed to B. Hihberti (Pal. Eossil. p. 612), viz. " close ver- 

 micular " ridges. Assuming that these ridges are in reality super- 

 ficial, and not portions of the deeper structure, there are only two 

 Carboniferous genera (to be described hereafter) whose scales 

 are of equal size, and present an approach to this kind of sculp- 

 ture. They are DendroptycMus and Rhomboptycliius. But in the 

 former the close-set ridges are subordinate to more distant larger 

 ones, in the latter they traverse conspicuous concentric grooves. 

 The specimens of Bhizodus which have occurred to me show as great a 

 variety in point of thickness as those oiHoloptycJiius, e. g. H. Andersoni 

 and H. glganteus. This difference of size may be specific ; but species 

 can scarcely be founded on isolated scales showing only their under sur- 

 face. The examples which revealed their upper aspect were as thick 

 as those of H. giyanteus, and, besides the undulating (not tuberculo- 

 linear) sculpture, had the anterior area also striated (not smooth), 

 drfferences which seem to be generic as between the Carboniferous 

 and Old Ped fishes. 



The surface of the pectoral arch and dentary bones is covered with 

 fine tubercles, close-set in small (apparently young) specimens ; but 

 in the large the tubercles are confluent by their bases, so as to cover 

 the whole surface with irregular pits or grooves bounded by ridges 

 of unequal height. A very fine pectoral arch (belonging to the 

 l^atural History Museum, Edinburgh), referred to this genus on 

 account of the presence of a doubly trenchant tooth, has around it 

 numerous fragments of fin-rays — long slender tubular rods of very 

 dense texture, similar to those observed in Dendroptychius. 



* The descriptions of Hibbert (Trans. Edin. Eoy. Soc. xiii. 24) are unfortu- 

 nately very meagre ; but there is, I think, reason for believing that the ornament of 

 Rhizodus, as above described, is intended in the figures of the scales, pi. 10. 

 figs. 1-3, the specimens figured come from the same beds as those to which I 

 have referred. 



