346 Prof. T. R. Jones and Dr. H. B. Holl on the 



two strong vertical furrows, extending from the back to the 

 ventral portion of the valve and dividing the surface into 

 three unsymmetrical lobes, transverse ridges, or bosses. 

 These vary considerably in size, mode of subdivision, and 

 relative position in different species and their varieties, and 

 to some extent in different stages of individual growth. The 

 hinder and usually larger lobe is somewhat leg-of-mutton- or 

 gigot-shaped ; hence it may be termed the " gigot-lobe." It 

 is often transversely sulcate. The lobe at the other end may 

 be called the " sausage-lobe," whether entire or constricted. 

 Its distended ventral extremity, making the extraordinary 

 " big lobe," is subglobular or apple-like. The middle lobe is 

 usually egg-shaped, but may be narrow and elongate. The 

 gigot and middle lobes are usually connected below, but 

 sometimes this condition is obscure ; and in some specimens 

 the ventral portions of all three lobes are almost equally 

 united. 



The ventral and two end margins of each valve are turned 

 sharply inwards^ the angle so made being marked externally 

 by a prominent rim or ledge, slightly rounded or trenchant, 

 along these three edges of the valve. An additional frill-like 

 free flange sometimes (in adults) stands out along the ventral 

 region * above the border. These thin laminar processes, as 

 well as the real marginal flange, are divergent, so as to give 

 a broad basis to the carapace standing on its ventral surface. 

 They are also sometimes frilled (crimped or goffered) with 

 radiate striae ; sometimes they have a spiny fringe, and fre- 

 quently present only denticles or short blunt spines. The 

 surface of the valves may be minutely pitted or coarsely 

 reticulate; and frequently small granulations, passing into 

 tubercles, ornament the whole or part of the valves ,• and the 

 large exaggerated antero-ventral lobe occasionally shows an 

 elegantly delicate reticulate surface. The dorsal edges of the 

 two valves meet along the hinge-line without any special 

 modification. The other contact-edges close together by the 

 marginal flange of the one valve being received by the edge 

 of the other. The valves are almost equal in size, the ven- 

 tral edge of the left valve very slightly overlapping that of 

 the right. 



Some special BeyrichicB^ typified by B. Wilckensiana 

 and its variety plicata, constitute the group Corrugatce'\. 



* This supernumerary flange is also met with in some Carboniferous 

 BeyrichicB and Kirkbyce. 



t Ann. & Mag. Nat. Hist. ser. 2, vol. xvi. pp. 89-90, 175, and vol. xvii. 

 p. 83, footnote. We now omit B. Bibeiriana, B. affinis, and B. Bar- 

 randiana {op. cif. pp. 170, 171) from the Corrugat(S. 



