STROPHOMENIDiE. 293 



OuTHis UNDATA, var. /3, M'Coy. Sil. Foss. Ireland, p. 36, pi. iii, fig. 21, 1846. 

 Strophomena deltoidea. Hall. Pal. New York, vol. i, p. 106, pi. 31 a, fig. 3, 1847. 

 LepToENA — M'Coy^ Brit. Pal. Foss., p. 234, 1852; and var. /3, undata, 



— p. 234, pi. 1 H, figs. 38 and 39, 1852. 

 Strophomena — Schmidt. Sil. Form. Ehstl., Archiv Nat. Liv., &c., vol. ii, 



p. 215, 1858. 



— — Salter. Mem. Geol. Surv., vol. iii, p. 269, 1866. 



Spec. Char. Shell semi-elliptical, deltoid, or semicircular, broader than long, widest 

 at the long straight hinge-line ; cardinal extremities angular ; front and lateral margins 

 rounded. Ventral valve geniculated, or very slightly convex, even almost flat to about 

 two thirds of its length, after which, by a sudden bend, it is abruptly arched and deflected 

 at an obtuse angle to the flattened plane of the posteiior portion of the valve ; area 

 triangular, about ten times as long as high ; fissure large, arched over by a pseudo- 

 deltidium. Dorsal valve following the curves of the opposite one, flattened posteriorly, 

 and abruptly bent upwards near the margin ; hinge-area narrow. Surface of both valves 

 marked by fine, equal, radiating, thread-like radii; a wide interspace, four or five times 

 as broad, intervening between each pair of ribs. This interspace is furrowed by five 

 or six finer striae ; and a slightly larger one, originating at variable distances from the 

 beaks, occupies the centre of the interspace. The flattened portion of each valve is 

 likewise marked by numerous concentric undulations or wrinkles, which are more strongly 

 defined as they approach the hinge-line. In the interior of the ventral valve a prominent 

 tooth is situated on each side of the fissure ; the saucer-shaped muscular cavity, extending 

 to about one third of the length of valve, is divided into two lobes, margined by a raised 

 ridge. In the interior of the dorsal valve there is a small bifid cardinal process, and an 

 anchor-shaped elevation, of which a wide median ridge forms the principal stem. On 

 either side are the muscular impressions, divided again by two smaller oblique ridges. 

 Length 10, width lo, depth 1 line. 



Obs. This is a very remarkable species on account of its well-marked internal 

 characters, which at once distinguish it from St. eiiglypha, St. funiculata, St. imbrex, 

 and others of its congeners. Externally it bears some resemblance to St. funiculata ; 

 but in this last the dorsal valve is the concave one, while in St. deltoidea it is convex, 

 and a similar difference exists in the ventral valves of both species. Prof. M'Coy observes, 

 at p. 234 of his work on ' British Palaeozoic Fossils,' that " This extremely variable 

 species can scarcely be defined. Some of our Coniston specimens have scarcely a trace 

 of transverse wrinkles, and have the profile gently arched from the beak to the length of 

 the visceral disk, whence the front is rather more abruptly arched ; the striae being 

 equal on the sides, but every 3rd, 5th, or 7th being larger than the rest on the middle 

 of the shell, agreeing exactly with Hall's figs. 3a and ?)d ; with this variety it seems the 

 Leptana complanata of Sowerby should be united. From these the passage is most 

 gradual, in specimens from the same locality, to those with a more abruptly deflected 



