OF SOUTnERN INDIA. 337 



7. Cucullaria, Desh., 1860, (Paris foss., 2ncl edit., p. 906). Elongated, sub- 

 oval, moderately inflated, radiately striated, inequilateral, beaks sub-anterior, in- 

 curved, very close together, the hinge area being very narrow and almost wanting 

 in some species ; hinge line moderately curved with a few shorter anterior and 

 some longer posterior fold-like teeth, arranged almost parallel to it ; numerous 

 sub-equal teeth are situated below the beaks; type. Area heterodonta, Desh., 

 from the Paris basin. Most of the species of this apparently well marked type 

 are from eocene deposits; its form agrees with Barbatia, as already noticed. 

 Deshayes describes a few species, like margaritula and decipiens, which should 

 also be placed in this genus, though he does not associate them with his 

 " CuGullaii'es." 



8. Cucullcea, Lam., 1801. Shell trapeziform, sub-quadrangularly elongated, 

 rather veutricose, generally somewhat inequivalve, sub-inequilateral, with the beaks 

 incurved and distant ; hinge area for the attachment of the ligament broad and 

 angularly striated ; hinge line straight, in the middle provided with a few small 

 cross or slightly oblique teeth, the outermost bending outwards and prolonged 

 more or less parallel to the hinge line; both, or only the posterior muscular 

 impression, are situated on an elevated plate ; type, C. concamemta, Martini. 



Laube (Denk. Acad., Wien, xxv, pt. ii), Strombeck and others have referred 

 ■a few triassic species to Cucullcea^ but the form of most of these agrees better 

 with Anomalocardia and Noetia than with the above-named arenus. Characteristic 

 species of Cucullcea, however, occm* in the Lias and Jura, and during the cretaceous 

 period their number appears to have been largest. Few are known from tertiary 

 deposits, and only three are found recent in the eastern seas. 



9. Trigonoarca, Con., 1867. Among the cretaceous forms there is a peculiar 

 group of species distinguished, the shells of which mostly resemble CucullcecB, 

 except that they are of a more solid structure and of a somewhat more oblique 

 form; their surface is concentrically and radiately striated, the concentric striae 

 being often more distinct than the radiating ones, which become occasionally 

 obsolete ; the posterior muscular impression is on an elevated plate, as in the former 

 genus, the hinge line is also quite similar to Cucullcea, but the hinge-teeth are very 

 numerous, and all are placed as if radiating from the centre. Those in the middle 

 are almost vertical, and towards either end they gradually increase in length and 

 turn more and more outward, retaining, however, their oblique position, at least on 

 the posterior side, while on the anterior one they occasionally become almost hori- 

 zontal. This character of the hinge very strongly recalls that of Noetia, and I 

 am not quite sure whether Trigonoarca should not be regarded as a sub-genus of it. 

 The shell of the former appears, however, to be often more elongated, less dis- 

 tinctly radiately costate, and the dental hinge line does not form an angle near the 

 middle, while this seems peculiar to Noetia. On the whole, Trigonoarca equally 

 resembles CucullcBa, as Noetia approaches Anomalocardia ; but in all the three last 

 named genera the hinge-teeth are either placed vertical to the hinge line or they 

 are descending on both sides, not ascending. 



