172 CARBONIFEROUS LAMELLIBRANCHIATA. 



Of P. pinguis, de Koninck, that author says (op. cit.), " Cette petite espece a 

 beaucoup de rapports avec la precedente [P. theciformis] ; elle s'en distingue par 

 ime epaisseur relativement plus considerable, raais surtout par la largeur 

 extraordinaire de sa facette ligamentaire et la convexite de son bord ventral." 



P. mytiloides, de Koninck, also seems to closely resemble the species under 

 consideration, but it is narrower in front than behind. 



Of British species, P. Oeinitzi is the one which is most like P. theciformis, but 

 it is relatively more transverse, and has very fine but microscopic radiating striae. 



Paeallelodon divisus, 1 M'Coy, sp., 1844 Plate XIII, figs. 11 — 14. 



Modiola divisa, M'Coy, 1844. Carb. Foss. Ireland, p. 74, pi. xi, fig. 30. 

 Abca divisa, d'Orbigny, 1850. Prodrome de Paleont., p. 134. 

 Modiola divisa, Morris, 1854. Cat. Brit. Foss., p. 209. 



— — Bigsby, 1878. Thesaurus Devonico-Carboniferus, p. 309. 



Avicula Hendehsoni, B. Etheridge, jun., 1878. Quart. Journ. G-eol. Soc, vol. 



xxxiv, p. 11, pi. i, fig. 11. 

 Modiola divisa, Kirkby, 1880. Ibid., vol. xxxvi, p. 589. 

 Avicula Hendeesoni, B. Etheridge, sen., 1885. Brit. Foss., pt. 1, Palaeozoic, 



p. 270. 

 Modiola divisa, B. Etheridge, sen., 1885. Ibid., p. 285. 



Specific Characters. — Shell small, compressed, transversely produced, somewhat 

 oblique. The anterior end is short and moderately tumid, narroAv in the 

 dorso-ventral diameter ; the anterior-superior angle a right angle. From this 

 point the anterior border curves rapidly downwards and backwards, and passes 

 without a break into the inferior border, which is faintly convex downwards, and 

 descends somewhat downwards and backwards, but curved upwards at the 

 posterior end, and indented in the neighbourhood of the byssal notch. The 

 posterior border descends from the right-angled posterior-superior angle and 

 curves forwards at first, then backwards, so that the upper part of the border is 

 concave from above downwards, but below, and towards the lower part, the border 

 is produced posteriorly, and is acutely rounded into the lower border. 



The hinge-line is straight, shorter than the transverse diameter. The 

 umbones are small, tumid, pointed inwards, hardly raised above the hinge-line, 

 non-contiguous, separated by a very narrow, ligamental area, and situated at the 

 junction of the anterior and middle thirds of the hinge-line. A rounded but well- 

 marked ridge extends from the posterior margin of the umbo towards the 



1 This species is not included in the list given on p. 130 ; for, although I have had the specimens 

 in my possession for some time, I did not recognise the generic characters until the preceding pages 

 were in print. 



