OF SOUTHERN INDIA. 117 



ligament situated on thickened but not prominent fulcra ; hinge with one 

 anterior, long, lamelHform tooth in each valve, bifid in the right, single in the 

 left valve, posterior cardinal teeth not distinctly traceable in either valve ; laterals 

 less distinct in the left valve. This is based upon the cretaceous Tellina str'ujata 

 of Goldfuss (see Petraef. Germaniae, pi. 147, fig. 18). In form it very much 

 resembles Moera, but the hinge presents some marked differences, as noticed 

 above. 



8e. Arcopagia, Leach, 1827, (teste Brown). There have been a number of 

 very different shells described under the name of Arcopagia, I shall here restrict 

 that name to those forms which are more or less closely allied to the type species, 

 Tell, crassa of Pennant. These are more or less sub-orbicular, with convex 

 valves, usually rough surface and always with a posterior plicature ; the cardinal 

 and lateral teeth are distinct ; of the latter those of the left valve are smaller than 

 those of the right, but usually distinctly traceable. Even as restricted in this 

 way there are two sections of shells of a somewhat different aspect ; the one 

 is almost orbicular like Tell, crassa, discus, and scobinata ; the other is j)Osteriorly 

 more produced, resembling the sub-genus TelUnula, like Tell, lingua-fells or 

 riigosa ; but the shells pass so gradually one into the other that no further separa- 

 tion appears advisable. Both forms are represented in tertiary and cretaceous rocks. 



8/. Linearla, Conrad, 1860, (Journ. Acad. Nat. Sc, Phil., iv, p. 279). Shell 

 elongated, sometimes roundish, not peculiarly thick, rounded on both ends, 

 surface partially or wholly radiately ribbed, posteriorly not, or very indistinctly, 

 fiexuous ; anterior cardinal teeth in both valves elongated, bifid, much smaller in 

 the left valve ; posterior cardinal small, but larger in the left than in the right ; 

 lateral teeth much thinner in the left than in the right valve, sometimes almost 

 obsolete in the former. This ought to include a large number of fossil species 

 which have been described as ArcopagicB ; the want of posterior flexure or 

 plicature and the usual radiate ribbings near the terminations of the shell parti- 

 cularly characterize those species. Among recent shells they are represented by 

 Tell, concentrica,* Gould, (not id. Reuss or d'Orb.), and one or two others. For 

 many years palaeontologists have separated these shells from Tellina and applied to 

 them the name Arcopagia. 



The hinge-teeth vary a great deal, sometimes the posterior cardinal tooth is 

 very elongated and laminar in the right valve, at other times it is nearly obsolete. 

 In the left valve the posterior cardinal is always short and thick, sometimes grooved 

 or bifid. 



Conrad instituted this genus for a North American cretaceous species, and most 

 of the known forms belong to the same geological period. Conrad's characteristic 

 is, however, in some respect u.nintelligible ; he says : " oval or oblong ; cardinal teeth 

 in the left valve two, the anterior one elongated, the other under the apex smaU and 

 bifid. This shell of which I have only the right valve belongs to a cretaceous group 

 which d'Orbigny has referred to the genus Arcopagia, &c." It is probable that the 



* I oulv know this species from the fignrain Chenu's Manual. 



2f 



