272 CARBONIFEROUS LAMELLIBRANCHIATA. 



postero-superior angle, where it is concave. This compression is continued 

 along and below the anterior portion of the hinge-line, forming a well-marked 

 excavation below the prosogyrous umbo, and takes the place of a lunule, which is 

 really absent. The antero-superior angle is well marked, and is a rounded right 

 angle, from which point the border descends in a regular, almost semicircular 

 curve, sweeping round and continuous with the curvature of the lower border 

 (which, however, is the segment of a larger circle than that forming the anterior 

 edge), and passes into the posterior border without a break. The latter is 

 regularly curved, larger than the anterior border; and it passes gradually into the 

 hinge-line above, so that the anteroinferior border forms one general curve of 

 varying intensity. The hinge-line is long, elevated, and almost straight in front, 

 slightly arched and produced posteriorly. 



The umbones are large, gibbose, produced and twisted forwards, with their 

 apices curled on themselves, pointing downwards, outwards, and forwards ; much 

 raised above the hinge-line and contiguous. 



The umbonal swelling expands rapidly, and forms the general convexity of 

 the valve, which is somewhat oblique in direction, the shell becoming gradually 

 less convex towards the borders. Parallel with the hinoe-line is a narrow 

 elongate groove for the external ligament ; it becomes shallower and broader 

 posteriorly. 



Interior. — The position of the anterior adductor muscle has not yet been 

 exposed, but the posterior is large and fairly deep, situated on the dorsal slope, 

 well within the margin. Pallial line simple. Casts show the usual elongated 

 groove for the posterior portion of the edentulous hinge-plate, and they show a 

 series of deep concentric regular grooves, — deeper, in fact, than are indicated on 

 the preserved exterior of the shell. 



Exterior. — The surface is covered with very numerous, fine, concentric lines 

 of growth, and on the anterior and posterior portions of the shell these are 

 thrown into a series of regular folds and sulci, which are almost obsolete over the 

 median portion of the valve and at the antero-superior angle. These folds are 

 narrow and close at the umbo, but become more widely separated and larger as 

 they approach the lower border. Shell thin. 



Dimensions. — PI. XXIII, fig. 5, the type specimen, measures — 



Antero-posteriorly .... 103 mm. 

 Dorso-ventrally .... 100 mm. 



From side to side, single valve . . .35 mm. 



Localities. — England : the Carboniferous Limestone of Thorpe Cloud and 

 Castleton, Derbyshire. Ireland : the Carboniferous Limestone of Malahide, St. 

 Dooghlas, co. Dublin ; also obtained in co. Limerick, but the locality was not 

 given. 



