OF SOUTHERN INDIA. 231 



impressions large, elongated, pallial line entire, soniewbat truncate posterior ; liga- 

 ment situated in a deep groove of the attached valve along the supero-posterior 

 margin, supported by a more or less thickened fulcrum and in the free valve attached 

 internally to the somewhat projecting margin. Type, Ch. frondosa, Brod. 



The sub-generic distinctness of Arcinella appears to be unsupported in the 

 structure of recent and fossil Chamce. Tlie- single cardinal tooth in the free valve 

 readily distinguished Chama from Monopleura. We have no distinct account of any 

 species of true Chama occuning in tlie Jurassic period, and even in the cretaceous, 

 there are only few species known with sufficient certainty, others more resemble 

 in form Diceras and JRequienia. In tertiary deposits several typical forms occur. 



7. Cnprina, d'Orb., 1823, (Flaglopti/chus, Math., 1812; vide Pal. Franc. 

 cret , vol. iv, p. 179; Zittel in Denksch. Akad. Wieu, 18G6, vol. xxv, pt. ii, p. 152). 

 Shell inequivalve, both valves with distinctly twisted, sub-spiral beaks, the lower 

 and generally larger valve is always attached when young, in full grown specimens 

 the place of attachment sometimes becomes more or less obsolete ; the hinge of 

 the attached valve consists of a more or less projecting and elongated sigmoid 

 cardinal tooth with a large pit in front of it and a small marginal one behind, in 

 which the corresponding two teeth of the free valve fit in ; the anterior muscular 

 impressions are on more or less elevated and rough prominences, the posterior 

 are larger, on a shallow excavated plate ; pallial line simple ; ligament external, 

 situated in a groove running in each valve from the beaks to the hinge-margin 

 and extending here along its edge. 



The shell consists at least of two distinct layers, the outer chiefly striated, being 

 very thick and often very porous, the inner (pyramidal) is more solid, and in large 

 specimens forms the interior laminse between the. chambers of the attached valve. 

 The free valve is said to be covered with an outermost, thin brown layer ; this is pro- 

 bably the thickened e])idermis which is thrown off in the lower valve on account of 

 the usual more lamellar structure. It does not appear to be a special layer, but the 

 outer surface of the striated one is only more solidified. There are also internally 

 some ribs occasionally present, and these are very variable ; sometimes there is only 

 one posterior, in other specimens an additional median one ajipears. 



Type, Caji. Anguilloni, d'Orb. Only few species of Caprina are known from 

 cretaceous rocks. 



Woodward (Manual, p. 150,) states that there is an internal cartilage present, 

 but there do not seem to be anyw^iere internally special grooves for it, and he 

 probably only alludes to the extension of the ligament along the inner edge of 

 the hinge line ; for in most of the recent Chamce the ligament is also to a certain 

 extent internal. The characters of the genus were ably described by Zittel in 

 Denksch. Akad. Wien, vol. xxv, pt. 2, p. 152, &c. 



8. Caprotina, d'Orb., 1812, (Pal. franc, terr. cret., iv, p. 236). Shell inequi- 

 valve, the attached valve is the larger, often radiately striated, while the upper is less 

 distinctly so or not at all ; the latter is usually moderately convex with a marginal 

 spiral beak ; hinge in the lower valve with a median cardinal tooth and a pit on 



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