156 CARBONIFEROUS LAMELLIBRANCHIATA. 



diameter posteriorly, equivalve. The anterior end is almost obsolete, and very 

 narrow from above downwards ; its border elliptically curved. The inferior border 

 is almost straight and descends rapidly downwards and backwards, meeting the 

 posterior margin in a very obtuse angle. The posterior border is obliquely truncate, 

 and straight above, but below is either polygonal or bluntly rounded. The hinge- 

 line is long and straight, forming an obtuse angle with the posterior border. 

 The umbones are small, obtuse, keeled, almost terminal, twisted forwards. Lunule 

 well marked. Escutcheon long and narrow. Proceeding from the umbo obliquely 

 towards the postero-inferior angle is a strong angular keel, dividing the valve into 

 two unequal triangles, the lower being much the smaller. Below this angular keel 

 the valve is compressed so that the surface is slightly concave. Above it the 

 dorsal slope is traversed by two sub-angular radiating ridges, which extend to 

 the margin, dividing the slope into three sub-equal parts. The hinge-line is 

 thickened, and immediately below it the dorsal slope shows a slight compression. 



Interior. — The anterior adductor scar is large and occupies almost the whole of 

 the anterior end, bounded posteriorly by a curved ridge. Normal in other details. 



Exterior. — The surface is ornamented by lines and ruga? of growth parallel to 

 the margins, interrupted on the dorsal slope by the radiating ridges. 



Dimensions. — PI. XXIII, fig. 16, a left valve, measures — 



Antero-posteriorly . . . .31 mm. 



Dorso-ventrally (posterior end) . . .17 mm. 



Transversely (estimated from a single valve) . 10 mm. 



Observations. — I have obtained four specimens of this shell from the shelly 

 limestone of Poolvash, Isle of Man, and I noted one other specimen in the collection 

 of the British Museum (Natural History). Sanguinolites monensis differs from all 

 the other species of the genus in its narrow, transverse, triangular form. It 

 resembles 8. striato-gramdosvs, but has no lines of radiating dots and is much more 

 cylindrical anteriorly. I can find no shell hitherto described which seems to have 

 the peculiar form and characters of that under discussion; nor have I yet seen 

 specimens from any locality except that noticed above. 



Sanguinolites vexillum, de Koiwicl", 1885. Plate XXIII, figs. 17 — 20. 



Sanguinolites vexillum, de Koiiinck, 1885. Ann. Mus. Roy. d'Hist. Nat. Belg ., 



torn, xi, p. 79, pi. xv, figs. 19, 31, 32. 



Specific Characters. — Shell below medium size, transverse, sub-trapezoidal, 

 narrower in front than behind, compressed. The anterior end is short and narrow, 

 its border a semi-ellipse. The inferior margin somewhat sinuous ; the posterior 



