STROPHOMENID^. 289 



Spec. Char. Semicircular, about as wide as long ; hinge-line slightly less than the 

 width of the shell ; cardinal extremities obtusely rounded ; dorsal valve geniculated, or 

 flattened to about half its length, after which the valve is abruptly bent with a rapid 

 curve, and at almost right angles to the flattened portion, with its sides slightly flattened 

 and rounded in front ; hinge-area very narrow, almost linear. Ventral valve concave, 

 flattened in its rostral portion, and following the curves of the opposite valve; area 

 triangular or sub-parallel, fissure narrow, covered with a pseudo-deltidium ; beak not 

 projecting. Surface of both valves ornamented with numerous, equal, thread-like radii, the 

 interspace between each pair of ribs four or five times as great as the width of each rib, while 

 three or four finer longitudinal radii occupy the interspace, becoming less numerous near 

 the beaks. In the interior of the dorsal valve the cardinal process is small, bifid, and 

 grooved ; and the quadruple impression of the adductor muscle is divided into two pairs 

 by a rounded median ridge. In the interior of the ventral valve the saucer-shaped 

 muscular depression is large, forming two oval elongated depressions, surrounded by a 

 raised margin, and between them is a smaller oval depression likewise divided in the 

 middle by a narrow median ridge. 



Length 22, breadth 22, depth 3 lines. 



Ohs. In 1847, while describing this species, I was the first to notice that the dorsal 

 valve is convex and the ventral one concave — the reverse to what we had observed to be 

 the case in Strophomena rhomboidalis. As stated by Prof. M'Coy, the Strophomena 

 {Lejjtana) imbrex of authors, which so closely resembles this species in general appearance 

 (of course, the geniculated variety), has the ventral valve convex, while in the shell under 

 description the same valve is concave, a similar difference obtaining in the other valve. 

 These modifications, as stated by M'Coy, were well seen from the figures published by 

 myself in 1847 and 1848. The interior surface of the valves, especially the rostral 

 portion, or that which surrounds the muscular scars, is strongly granulated. In the 

 young state the muscular impressions are very feebly defined. Almost invariably this 

 (and every species) appears to be thickened considerably round the margin of the shell in 

 advanced age, or when the animal has attained its full period of development. 



Position and Locality. I am acquainted with Strophomena eucjlypha from the 

 Llandovery, Wenlock, and Ludlow formations. 



In the Lower Llandovery it occurs at Sevin, Llettyrhyddan, Llandovery, where it was 

 collected by Mr. Hughes; and Prof. Ramsay obtained it at the Quakers' Burial Ground, 

 Welsh[)Ool. At Penlan, Llandovery, it occurs in the Upper Llandovery. In the 

 Woolhop'C Limestones at Littlehope. In Wenlock rocks near Llansannan, North Wales 

 (beds above the Denbighshire grits). It is also abundant at Wenlock, Benthall Edge, 

 Dudley, and several localities in the Malvern, Abberley, Woolhope, andUsk districts. In 

 the Lower Ludlow of the Abberley and Usk districts. In the Aymestry Limestone at 

 Dormington Wood, and at Leintwardine, Shropshire, &c. In the Upper Ludlow, at Hay 

 Park, south-west of Ludlow, Chance's Pitch, Malvern, &c. 



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