320 



CRETACEOUS LAMELLIBRANCHIA. 



I. latus, Man tell (Fig. 76) is a large, slightly convex form in which the folds 

 have almost disappeared. In the type (Fig. 76) the postero-dorsal part of 

 the valve is missing, so that in Mantell's figure the umbonal part of the valve 

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



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

 is often of large size. The angle formed by the anterior margin and the hinge- 

 line is rather larger than usual, and an anterior ear may be developed ; other forms 





Pig. 78. — Inoceramus Lamarcki var. Cuvieri, Sow. Upper Chalk, Lewes. Sedgwick Museum, Cambridge. 



Eight valve. Natural size. 



are similar (Fig. 79), 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, 

 but with or without folds, may be termed /. Lamarcki var. Guvieri (Plate LIII, 

 fig. 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 found separately. It is thickest 

 near the umbo, and becomes thinner towards the posterior end. The ligament 

 pits are numerous, shallow, and two, three, or more times higher than long, 



