STROPIIOMENA. 125 



strong, radiating, rounded striae with flattened interspaces of variable width, partly occupied 

 by one or two smaller stria), the ribs and interspaces being, at the same time, closely 

 intersected by fine concentric lines or striae, giving to the longitudinal ones a crenulatcd 

 appearance. 



In the interior of the ventral valve, a strong hinge-tooth is situated on either side at the 

 base of the fissure, and strengthened by small dental or rostral plates ; the muscular im- 

 pressions form a saucer-shaped depression, partially surrounded by a slightly elevated ridge ; 

 the adductor or occlusor (a) occupies the central portion, and forms two small elongated 

 depressions, separated by a slightly elevated mesial ridge, and on either side are the larger 

 scars (r), apparently composed of two parts, the anterior or central being due to the 

 divaricator, while the other or outer one would be produced by the ventral adjustor ? In 

 the interior of the dorsal valve, the cardinal process (j, to which were attached the 

 divaricator muscular fibres, PI. 27, fig. 6, 7) is composed of two testaceous projections ; the 

 socket plates are large, and partially united to the lower portion of the cardinal process. 

 Under these, on the bottom of the valve, may be seen the quadruple impressions left by 

 the occlusor, and which occupy above one third of the length of the valve, and are arranged 

 in pairs divided by a short rounded median ridge. 



_ Dimensions and relative proportions very variable ; some examples have attained or 

 exceeded three lines in length, by four in width, the depth varying also from a few lines to 

 about two inches. 



05s. Several palaeontologists have already coincided in the belief that S. senilis is 

 nothing more than a different state of S. crenistria, viz., in which the shell has attained an 

 excessive depth or degree of convexity, accompanied by a very large area. Professor M'Coy 

 justly states S. Sharpei to be an undoubted synonym ; and he might have said as much of 

 0. Bechei, for the so-termed species has been made out of a crushed imperfect cast or 

 impression of the same derived from the Carboniferous slate of Whiling Bay, Younghall, 

 Ireland, as may be seen from an inspection of the original specimen in Sir R. Griffith's 

 collection. 0. caduca is a small flattened valve of S. crenistria, or of var. arachnoidea ?, 

 the original specimen in Sir R. Griffith's possession being derived from a black Carboniferous 

 slate at Rahoran, Fivemiletown, Ireland. 0. comata is founded on a fragment of dorsal 

 valve ; for the original specimen in Sir R. Griffith's collection shows the cardinal process, 

 although not represented in the ' Synopsis ;' the striae are also not so close and regular as in 

 the Irish author's enlarged illustrations, but agree more with those of S. crenislria, to which 

 the specimen should be referred; and I cannot help feeling somewhat surprised that any author 

 could fabricate a species upon such insufficient material as the third part of single valve 

 must naturally be. In his work on the ' Cambridge Palaeozoic Fossils,' while describing 

 S. crenistria, Professor M'Coy justly observes that " the striation is very variable, according 

 to the state of preservation : in some the principal striae, being nearly a line apart, and 

 the intervening flat spaces having very distinct, longitudinal, fine lines, internally punctured, 

 the middle one largest and crossed by fine, close, deep, irregularly transverse wrinkles ; in 



