LEIOPTERIA GRANDIS. 11) 



Leiopteria gkandis, sp. nov. Plate III, figs. 12 — 15. 



Sperijic (Characters. — Shell above medium size ; compressed, arcuate ; very 

 inequilateral ; posterior inferior extremity much produced, flattened and expanded ; 

 aviculoid. 



The anterior end is very small, almost obsolete, marked off from the shell l)y a 

 byssal groove. The inferior border is very convex and extensive, sweeping round 

 behind to join the posterior margin, which is lunate and also long. The hinge-line 

 is short and straight, only very slightly produced behind the posterior margin. 

 The umbones are very small, narrowed and flattened, not raised, and placed very 

 far forwards, but not terminal. The body of the valve is Innately tumid above, 

 expanded and flattened below and behind. The posterior wing is comparatively 

 small, triangular, and much flattened, only slightly marked off from the convexity 

 of the valve by an ill-defined groove. 



Interior. — Unobserved. 



Exterior. — The surface is covered with flattened subimbricating lamellae, lines, 

 and striae of growth. 



Dimensions. — PI. Ill, fig. 14, from Castleton, measures — 



Antero-posteriorly . . . .118 mm. 



Dorso-ventrally .... 05 nnn. 



From side to side (estimated) . . .25 mm. 



ZocaUtips. — England: the Carboniferous Limestone of Withgill, Yorkshire; 

 Thorpe Cloud and Castleton, Derbyshire. 



Observations. — This species is very common at Treycliff, Castleton, and also 

 occurs at Cavedale Quarry. It is difficult to obtain perfect examples, owing to the 

 size and flatness of the shell. Specimens in all stages of growth are to be found, 

 but none have yet been obtained with the valves in contact. 



The species is characterised not only by its size and flattened form, ])ut by the 

 smallness of the posterior wing and absence of the rostrum. PI. Ill, fig. 14, from 

 Cavedale Quarry, shows the peculiar shape of the adult, but unfortunately is 

 incomplete at the umbo and anterior end. PI. Ill, fig. 12, is that of a right valve, 

 incomplete below, but showing the hinge-line and posterior slope. It is at present 

 impossible to state whether the shell was equivalve or no. 



Probably this species has been referred to Pterhwa lxviijata,WCoy,h\\t it does 

 not possess the well-marked oblique ridge, and has a comjiaratively much smallei- 

 flattened posterior ear and no rostrum. 



