320 



CllETxVCEOUS LAMELLlBliA.XCUIA. 



/. hitns, ^Mantel] (Fig. 76) is a large, slightly convex form in wliicli the folds 

 have almost disappeared. In the type (Fig. 7')) the postero-dor.'^al part of 

 the valve is missing, so that in ]\ranteirs figure the umbonal part of the valve 

 appears to be more acute than it really is (compare Fig. 75). 



T. Guvieri, Sowerby (Fig. 73) is only slightly convex, with indistinct folds, and 

 is often of large size. The angle formed by the antei'ioi- margin and \\\v hinge- 

 line is rather larger than usual, and a!i anterior ear niav be developed ; other forms 



Via. 78.— /iiocccamns Lamarcki var. Cuvieri, Sow. Upper Chalk, Lewes. Sedgwick Musouin, CambiiJfjo. 



Eiglit valve. Natural size. 



are similar (Fig. 7'.'), but have a smaller angle between the anterior margin and 

 the hinge, and these pass into forms with more distinct and eventually with 

 strong folds (Figs. 77, 78, 82). The varieties similar in form to Sowerby's type, 

 l)ut with or withont folds, may be termed /. Lamarcld var. Cuvieri (Plate LIII, 

 tig. 7, Text-figs. 73 — 84). The hinge in large specimens of this variety (Fig. 80) 

 is of great thickness, and portions of it are often founil separately. It is thickest 

 near the undx), and l)ecomes thinner towards the posterior end. The ligament 

 pits ai-e numerous, shallow, and two, three, or more times highei' than long, 



