the Palaeozoic Bivalved Entomostraca. 215 



more neatly ovate in outline, and narrower behind. The surface 

 of the carapace is smooth and regularly convex, but impressed 

 distinctly with the two sets of notches, of which the anterior 

 is liable to disappear in Th. corjmlenta. The caudal notch 

 consists of two oblique pits, usually meeting posteriorly, and 

 forming a subtriangular depression, V-shaped, with the apex 

 pointing backwards, but not reaching the border. The ante- 

 rior notch is just in front of the middle of the valve, transverse, 

 and equal in length to about a third of the valve's height (or 

 breadth) . 



Only single valves have been found. The figured specimen 

 is neatly ovate, smoothly convex, with the hinder notch 

 chevron-shaped ; suboblong in profile, and nearly circular in 

 end view (if the valves were united) . 



Rare in the Wenlock Limestone of Croft's Quarry, near 

 Malvern. 



This form is not at first sight very dissimilar to some larger 

 and more convex specimens, from the base of the Wenlock 

 series ; but the latter have the single caudal furrow of 

 Thlipsura covpulenta, they are smaller and neater, and in 

 one instance the transverse anterior notch is plainly seen, 

 though not so well defined as in the little species before us. 

 We have already mentioned that one of the larger specimens 

 (from the Woolhope beds) shows a trace of the anterior notch. 



It may be that the chevron notch is lost by the growth of 

 the individual, one of its arms enlarging into the great single 

 furrow of the large forms ; but we have not yet found any in- 

 termediate stage of growth, and therefore propose to recognize 

 the small form as a distinct species, which has both the ante- 

 rior and posterior notches notably distinct and peculiar. 



Cytherellina, gen. nov. 



In the Upper Silurian strata both of Gothland and Britain 

 occur numerous transversely indented casts of little subtrian- 

 gular Bivalved Entomostraca, to which one of us, in 1855, 

 gave the name of Beyrichia siliqua, under the supposition that 

 some of these bisulcate casts (in Scandinavian limestone) were 

 Beyrichiw, though of unusual aspect. The supposed " mar- 

 ginal rim " was supplied by the broken edge of the imbedded 

 shell, as will be understood by the reader if he looks at figs. 6 

 a, b, c, d, upside down, comparing them with the older figure, 

 Ann. Nat. Hist. ser. 2. vol. xvi. pi. 5. fig. 22. The appear- 

 ance of the calcareous cast was sufficiently deceptive to mislead 

 until specimens were found in a different limestone (from near 

 Malvern), which presented both outer tests and internal casts, 

 of different tints and texture, and indubitably related to each 



