MOLLUSCA. LAMELLIBRANCHIA 221 



cardinal teeth. Adductor impressions unequal. Pallial line 

 simple. Cretaceous to present day. Ex. V. planicosta, Bracklesham 

 Beds. 



Chama. Shell irregular, thick, inequivalve, fixed by the 

 umbo of the larger valve (generally the left, sometimes the right). 

 Umbones spiral or sub-spiral, directed anteriorly, that of the fixed 

 valve longer than the other. Surface with concentric lamellae or 

 spines. The fixed valve larger and much deeper than the other. In 

 each valve a strong cardinal tooth, and sometimes in the inferior 

 valve a narrow curved posterior tooth also. Ligament external, in 

 a deep groove, prolonged towards the umbones. Adductor impres- 

 sions large, the anterior commencing near the hinge-line. Pallial 

 line simple. Upper Cretaceous to present day. Ex. C. squamosa, 

 Barton Beds. 



Diceras. Shell thick, inequivalve, fixed by umbo of larger 

 valve. Umbones large, inrolled, directed forwards. Ligament 

 external, in a curved groove at the posterior margin of the hinge. 

 Hinge-plate very thick ; right valve with a large, elongate, curved 

 posterior cardinal tooth parallel to the ligament groove, and an 

 anterior pit ; left valve with a large ear-shaped cardinal tooth and 

 a pit for the tooth of the right valve. Adductor impressions 

 distinct, the posterior on a raised elongated plate. Pallial line 

 simple. Upper Jurassic. Ex. D. arietinum. Requienia and Toucasia 

 (Cretaceous) are related to Diceras. 



Hippurites (figs. 96, 97). Shell very large and massive, conical 

 or sub-cylindrical, not spiral, very inequivalve, fixed by the apex of the 

 larger valve. The large {lower) valve elongate-conical, striated or 

 smooth, and with three parallel furrows extending from the apex to 

 the cardinal margin, due to folds of the shell-wall which give rise to 

 three corresponding ridges in the interior. Hinge consists of a small 

 cardinal tooth and of cardinal pits ; anterior adductor impression 

 large and divided into two separate parts ; posterior adductor in a 

 depression. Small {upper) valve flattened or slightly convex, 

 operculiform, porous, the pores leading into canals ; with a central 

 umbo and two prominent teeth ; the anterior tooth very large with 

 two surfaces at its base for the attachment of the adductors ; the 

 posterior tooth smaller with a tooth-like process for the posterior 



