232 CARBONIFEROUS LAMELLIBRANCHIATA. 



the name Amphidesma PortlocJcii to the shell originally figured as A. axiniforme, 

 Portlock. 



This species varies considerably in the length of the hinge-line posteriorly, a 

 variation which is accompanied by a change in the posterior slope and the apparent 

 length of the posterior border. In large examples the inferior border seems to 

 become sulcated towards its posterior end, giving a beaked appearance to the 

 postero-inferior angle, and as in figs. 10 and 19, PI. XVII. P. axiniformis is much 

 less convex than P. rectangularis, and it also lacks the compressed antero- and 

 postero-superior angles of the latter species. It is less oblique than P. obliquus, 

 and has its posterior dorsal slope less obliquely truncate. P. impressus is more 

 transverse, and the posterior umbonal ridge is less angular. 



P. axiniformis seems to have the largest vertical and horizontal distribution of 

 all the species of the genus. It is not met with in purely organic limestones, but 

 only in shales or earthy calcareous beds. 



Protoschizodtjs axiniformis, var. depressus, Portlock. Plate XVII, figs. 11, 12. 



Amphidesma depeessa, Portlock, 1843. Geol. Rep. Londonderry, p. 439. 

 — caebonabta, Portlock, 1843. Ibid., p. 438, pi. xsxvi, fig. 8. 



Varietal Characters. — Shell compressed, transversely suborbicular, sub- 

 equilateral. The anterior end is markedly produced, and has a broadly 

 semicircularly curved border, which passes gradually into the lower border, which 

 is more broadly convex. The posterior border is bluntly rounded, with an 

 approach only to angulation above and below. The hinge-line is arcuate. The 

 umbones are small, comparatively broad and flattened, pointed, close, and subcentral. 

 The valves are flattened, and there is only an obscure trace of an oblique line on 

 the slightly compressed dorsal slope. 



Exterior. — The surface is covered with regular, scarcely visible, concentric 

 lines of growth. Shell thin. 



Dimensions. — The type of Portlock's Amphidesma depressa, PI. XVII, fig. 12, 

 measures — 



Antero-posteriorlv . . • .23 mm. 



Dorso-ventrally . . . .18 mm. 



Locality. — Carboniferous shales of Clogher, co. Tyrone, Ireland. 



Observations. — Although described, the shell named Amphidesma depressa was 

 not figured in Portlock's work. It shows the extreme degree of variation, 

 Portlock's A. carbonaria coining midway between it and P. axiniformis. The 

 variety differs chiefly in the marked growth of the anterior end, so that it 



