1866.] YOUNG CAEBONIFEROFS GLYPTODIPTERINES. 599 



Rhizodfs, Owen {=Apepodus, Leidy)*. Figs. 4 & 5. 



The teeth of two sizes and trenchant on both edges, on which 

 this genns is founded, and its distinctness from Holoptychius and 

 Megaliclithys established, are accompanied by scales which are dis- 

 tinguished from those of the latter genus by the fact that their 

 anterior area is cycloidal and concentrically striated, not smooth and 

 rhombic. The free area, on the other hand, is covered with coarse 

 tubercles, and has a general resemblance to that of Holoptychius. 



No fragments have yet been found from which the shape of the 

 body or the structure of the head can be determined. One or two 

 varieties of the teeth occur ; thus one edge only may be trenchant, 

 the posterior (?) being evenly rounded. Of such teeth some are 

 angiiated more or less sharply, their section passing from triangular 

 with rounded angles to polygonal. Kone of the varieties have been 

 named, as it is possible that they may be individual differences ; 

 further, the determination of several teeth hitherto isolated has pro- 

 ceeded so rapidly that it is better to wait for more materials. 



Leidy (Journ. Acad. Nat. Sc. Philadelphia, iii. 1855-58) de- 

 scribed and figured a tooth of a Carboniferous genus Apepodus, be- 

 tween which and Rhizodus, as known to British palaeontologists, no 

 substantial difference can be detected. The same beds, said to be of 

 Carboniferous age, yielded scales which Leidy figures as Holopty- 

 cJiius Americanus ; their characters are very similar to those of H. 

 giganteus, Agassiz (Poiss. Vieux Gres Eouge, pi. 24. f . 3-10). Whether 

 these beds be of Carboniferous age, as affirmed by Leidy, or Upper 

 Old Red, the association of the two genera in the same deposit is an 

 interesting fact not yet paralleled in this country. 



The scale figured is taken from a large block of limestone from the 

 Gilmerton quarries near Edinburgh. A similar block is so placed in 

 the rooms of the Eoyal Society there as to render its examination a 

 task of difficulty. The scales are associated, on it, with typical teeth of 

 Rhizodus, and with fragments of pectoral arches and other portions 

 of the skeleton. 



The scales are rotundo-quadrate, the anterior margin flattened, 

 the posterior slightly produced. They are thick, bony ; their under 

 surface is marked by concentric growth-lines, and bears a large or 

 small, but constant, subcentral boss. The overlapped, anterior area 

 of the upper surface is never less than a moiety of the whole ; it is 

 covered with fine concentric and a few radial striae. The free surface 

 is thrown into broad undulations, which are approximately parallel 



* It is commonly stated that Megalichthys Hibberti is the synonym of Ehi- 

 zodus Hibberti. Dr. Hibbert's paper, which is as remarkable for its rare gene- 

 rosity as for the extent of the research it sums up, contains a passage which shows 

 that Agassiz had not wholly overlooked the different forms of the teeth, though 

 he afterwards underrated their importance. Agassiz, it is said, proposed the 

 name M. falcatus for a form of tooth to be distinguished from that of Burdiehouae 

 by being very sharp on the edges. Bhizodus is abundant in the Burdiehouse 

 beds. The assertion, therefore, of a difference as well in locahty as form can 

 only be explained by the rapidity with which the great naturalist passed from 

 place to place, and thus, while he increased the materials for his scheme of classi- 

 fication, slipped into minor errors of detail. 



VOL. XXII. PAET I. 2 S 



