UNIONIDiE. 251 



its antero-superior angle. This species is more regularly oval and less obliquely 

 triangular than most of the others included in the genus, and it may be distin- 

 guished from P. sequilateralis by the absence of an oblique angular keel, also by 

 its comparatively larger anterior end, and the absence of any general obliquity. 



R. Btheridge, jun., described certain shells under this species, but these I now 

 place with P. sequilateralis for reasons given above. In his observations on the 

 more perfect of these specimens Mr. Etheridge admits that " the obtuse diagonal 

 ridge is more pronounced, the posterior portion of the ventral margin straighter." 

 It is not to be wondered at, considerable uncertainty having existed as to M'Coy's 

 species, because the specimens were so poor and the descriptive accounts very 

 meagre, but a study of the type makes it impossible to associate with it forms 

 which have an acute oblique ridge, anterior prosogyrous umbones, and a more 

 generally flattened shell, such as is characteristic of P. sequilateralis. 



P. subtruncatus appears to me to be more closely allied to P. fragilis ; but it is 

 flatter, and has not the squared subalate posterior angle ; it is quite possible, 

 however, that the one may only represent the adult form of the other. Owing to 

 the paucity of examples it is impossible to pronounce definitely. 



Family UNIONISE. 

 Genus Carbonicola, M'Coy, 1855. 



(See ' Monograph on Carbonicola, A.nthracomya, and Naiadites,' Pal. Soc, 1894, p. 38.) 



Generic Characters. — Bquivalve, inequilateral, transversely ovate, gibbose ; scar 

 of accessory pedal muscle above that of anterior adductor. Hinge-plate triangular, 

 with or without cardinal teeth ; no anterior and posterior lateral teeth. Pallial 

 line non-sinuate. Periostracum thick, wrinkled ; ligament external ; umbones 

 often eroded. 



Species. 



Cabbonicola eobusta, Sowerby, sp., 1840. Monogr., 1894, pp. 45, 174, pi. i, 



figs. 1 — 6 ; pi. ii, figs. 1 — 6, 9 — 11 ; pi. xxi, 

 figs. 11, 11a, and 12. 



— kugosa, Broivn, sp., 1843. Ibid., p. 49, pi. ii, figs. 7, 8, 8 a. 



— acuta, Sotv., sp., 1829. Ibid., pp. 150, 175, pi. iii, figs. 1 — 12 ; 



pi. iv, figs. 8—17; pi. v, figs. 1, 3—37, 39; pi. 

 vi, figs. 1 — 43 ; pi. xxi, fig. 1. 



