920 DR F. R. C. REED ON THE 



hinge-area to the pedicle-valve, and concentric lamellose growth-ridges. This 

 species is named after Professor Wyvillb Thomson. 



Hemipronites Thomsoni, var. nov. inconstans. 

 (Plate XXI, figs. 29-31.) 



1883. Orthis elegantula, Dalman, Davidson (pars), Mori. Brit. Foss. Brack., vol. v, Silur. Suppl., 

 pp. 178, 226. 



Shell subcircular, subquadrate, or transversely elliptical, unequally biconvex ; 

 hinge-line rather less than maximum width of shell ; cardinal angles obtuse, 

 rounded or subrectangular. Pedicle-valve convex, subcarinate in young, deepest 

 near middle ; beak large, elevated, rising high above hinge-line, acute, slightly 

 incurved ; hinge-area large, high, triangular, slightly concave, nearly half the length 

 of shell in height, lying nearly in plane of valve, with narrow triangular delthyrium. 

 Brachial valve much shorter and less convex than pedicle-valve, transversely elliptical 

 in shape, more or less bilobed by a sharp median depression which extends from 

 beak to front margin, increasing in width and causing sinuation of edge of valves ; 

 beak small, obtuse, rising slightly above hinge-line, slightly incurved, with narrow 

 hinge-area below, set at right angles to that of other valve. 



Surface of valves covered by numerous small rounded somewhat unequal closely 

 placed riblets curving back a little on each side and numbering about 60 on the 

 margins, about one-third of which are primaries, the others (usually rather smaller) 

 arising somewhat irregularly by intercalation at about one-third or one-half their 

 length. Concentric growth-ridges usually few, lamellose, strong, submarginal. 



Dimensions. — Average length, 9-11 mm. ; thickness, 6-8 mm. 



Horizons. — (l) Stinchar Limestone Group; (2) ? Balclatchie Group. 



Localities. — (l) Craighead ; (2) Balclatchie (conglomerate). 



Remarks. — This small shell, of which only a few specimens from Craighead are 

 positively known, appears to belong to the group of Hemipronites \_Orthisina] injlexa, 

 Pander.* The internal characters are unknown. It differs from H. carrickensis by 

 the strongly marked bilobation of the brachial valve, by its sinuated anterior margin, 

 by the decided carination of the pedicle-valve, and by its relatively higher area. 

 The ornamentation is quite unlike that of Scenidium Lewisi, var. craigense, though 

 the somewhat similar shape of the shell might suggest a comparison. The re- 

 semblance to H. Thomsoni is, on the other hand, so close that we must regard it as 

 a variety of this species, distinguished only by its marked carination and bilobation. 



Genus Scenidium, Hall. 

 Scenidium Greenoughi, sp. nov. 

 (Plate XXI, figs. 32-34.) 

 Shell semielliptical to subtriangular, widest along hinge-line ; cardinal angles 

 less than right angles. Pedicle-valve convex, deep, swollen, subcarinate ; beak high, 



* Lamansky, Mem. Com. Ge'ol. St Petersbourg (N.S.), vol. xx, 1905, pp. 73, 176. 



