STROPHOMENID^. 319 



Lept^ka transversalis, Schmidt. Sil. Form. Ehstl. ; Archiv, &c., p. 216, 1858. 



— — B'Eichw. Lethaea Rossica, vol. i, p. 869, 1859. 



— — Salter. Siluria, 3rd ed., pi. ix, fig. 17 d, and pi. xx, fig. 17, 



1859. 



— — Lindstrom. Gotland's Brach., p. 374, 1860. 



— — Salter. Mem. Geol. Surv. Scotland (sheet 32), p. 138, pi. 



ii, figs, 8 and 9, 1861. 



— — Haswell. Sil. Form. Pentlands, p. 35, pi. ii, fig. 17, 1865. 



— — Salter. Mem. Geol. Surv., vol. iii, pp. 276, 277, 1866. 



— — Dav. Geol. Soc. of Glasgow, Pal. Trans., vol. i, p. 19, 



pi. iii, fig. 13, 1868. 



— — Bigsby. Thesaurus Siluricus, p. 97, 1868. 



Spec. Char. Semicircular, or transversely fusiform, wider than long ; hinge-line 

 nearly or quite as long as the shelPs width ; cardinal extremities obtusely or acutely 

 angular, but more often short, with rounded semicylindrical ears. Ventral valve very 

 convex, convoluted at the beak, which is sometimes large and greatly incurved ; area 

 narrow ; fissure partly arched over by a narrow, convex pseudo-deltidium. Dorsal valve 

 concave, following the curves of the opposite valve ; hinge-area very narrow, almost linear, 

 with a projecting trilobed cardinal process in the middle. Surface of both valves covered 

 with radiating, thread-like, equal-sized riblets, about half a line apart near the margin, 

 the interspace between each pair being occupied by from five to seven very fine parallel 

 striae. In the interior of the ventral valve is a prominent tooth on each side of the fissure ; 

 the muscular impressions forming two more or less elongated, deviating, oval depressions. 

 In the interior of the dorsal valve, under the trilobed cardinal process, at about one third 

 of the width of the inner surface is occupied by the muscular impressions or projections, 

 which are divided into three almost equal portions. The central division is occupied by 

 two rounded longitudinal ridges, with a smaller one in the centre (fig. 7), while on either 

 side the muscular scar is surrounded by a raised ridge, the interspace between its sub- 

 parallel sides being occupied by a projecting rounded ridge. There exists also a sub- 

 convex margin around the lateral and frontal portion of the valve. Vascular impressions 

 are visible near the margin in both valves, as well as numerous rows of tubercules, which 

 give the cast a punctured appearance. 



Proportions varying according to the specimen. 



Length 9, width 11, depth I line. 

 Obs. This is a well-known and rather common species ; and appears to have been 

 first noticed by Wahlenberg, who in 1821 described it in the following few words — 

 " Anomites transversalis laevissimus semiorbicularis, cardine rectiiineo indiviso latitudinem 

 testae excedente ; in Gothlandia et Osmundsberg Dalecarliae proveniens, diu A. truncatus 

 audivit. Diff'crt autem certissime cardinis linea in media testa non interrupta, laevitate 

 superficiei exterioris," &c. It was not figured, however, until 1827. It is a very variable 

 shell ; and, as remarked by M'Coy, it is considerably more globose than Leptcena sericea. 



