LEPTODOMUS. 75 



1. Leptodomus constricta, M'Coy. Plate VIII, figs. 8, 9. 



184-1. Ctpeicabdinta impeessa ? Phillips (not Sowerhy). Pa Toss., p. 36, 



pi, xvii, fig. 58. 

 1855. Leptodomits coj^steictxis, M'Coy. Brit. Pal. Eoss., p. 39G, pi. Ha, 



fig. 10*. 



Description. — Left valve large, transverse, subquadrate, convex. Umbo very 

 large, wide and prominent, depressed in the centre, much elevated above the hinge- 

 margin, situated at about the median third of the length. Hinge-margin long, 

 slightly concave behind the umbo. Anterior margin arched, being oblique from the 

 umbo to the anterior corner, which is rapidly rounded, and then oblique and 

 moderately convex in its inferior part. Inferior margin long, slightly oblique, and 

 convex but depressed in the centre. Posterior margin roundly convex, straighter 

 and oblique above, and rounding into the hinge-margin. Lunule apparently long, 

 lanceolate, excavate, bounded by a straight elevated line from the front of the 

 umbo to the anterior point. Contour of back convex, with a wide, well-marked, 

 subangular depression (running from the middle of the umbo straight and 

 obliquely to the centre of the inferior margin) from each side of which the contour 

 rises with a gentle convexity, being bounded laterally by the ridge of the lunule in 

 front, and sloping gradually to the margin behind. Surface covered by about eleven 

 or twelve large, low, rounded transverse ridges, which divaricate from the front, 

 are lost in the median constriction, become again prominent for a short distance, 

 and then vanish on the posterior part of the shell. 



Size (of left valve). — Length 50 mm., height 33 mm., depth 11 mm. 



Localities. — A fine specimen from Marwood is in the Museum of Practical 

 Geology; M'Coy's type specimen and another small specimen from Marwood, and 

 a poor specimen from Top Orchard, are in the Woodwardian Museum ; another 

 from Roborough is in the Porter Collection. 



Bemarhs. — This is evidently the species to which M'Coy has given the name 

 L. constricta, when describing its right valve from three specimens from Marwood.^ 

 He was inclined to think it the same as " the shell referred by Phillips (in the 

 ' Pal. Foss.') to the Silurian Gypricardia impressa of Sowerby," and this, I think, 

 our two figured specimens prove undoubtedly to be the case. 



Gypricardia ? impressa, Sowerby,^ itself is probably closely allied, but it seems to 

 be smoother and more transverse, and to have a smaller and more anterior umbo. 



Affinities. — It appears to me that the present species is closely allied to the 

 German shell described by F. Romer as Myacites impressus^ but specifically differs 



1 1839, Sowerby, in Murchison's ' Sil. Syst.,' p. GOO, pi. v, fig. 3. 

 ^ 1844, P. Eomer, * Ebein. Uebergangsgeb.,' p. 79, pi. ii, fig. 4. 



