68 CRETACEOUS PELECYPODA 



and the same species. It is always difficult to give a decided opinion on specimens 

 which are imperfect, but my suggestion as to their identification is strengthened 

 hy our specimens, which certainly are Forbes' M. tripartita, and appear to be from 

 the same locality as the specimen named by E or be s' Curdium intersectum ; at least 

 the yellowish brown calcareous rock is in both cases the same. There is no better 

 conclusion to be derived from Eorl)cs' original, and we should only be justified in 

 recognizing it as distinct upon the discovery of well preserved specimens which agree 

 with it in form better than with M. tripartita ; taking, however, our present mate- 

 rials as a basis, such forms as represented in figure 9 6 (j)l. V) cannot be acknow- 

 ledged as inde2)endent species. 



Localit!/. — About two miles north of Alundanapooram and three miles south of 

 Serdamungakun ; apparently not common. 



Formation. — Trichinopoly group. 



V. Family— AN ATIN IB ^. 



As regards the form of the animals the Aijatividm offer such a large amount of 

 variation, that it will be better to point out these diflTerences when speaking of the 

 characters of each sub-family. All of them, however, have the mantle margins 

 united, with an opening at the antero-inferior side for the protrusion of a small 

 digitiform foot ; the siphons are more or less prolonged, united in their entire length 

 or only at the base, and the united portion of these siphons is almost invariably 

 covered with an extension of the epidermis which covers the shell. There is also 

 a small opening in the mantle below, at the base of the siphons. The gills are 

 thin and in many cases (though not invariably) single. The palpi are usually long 

 and narrow. 



The shell is generally thin, consisting of two usually well defined layers, the 

 outer being opaque, calcareous, very often punctated or finely granulated, the inner 

 pearly ; the latter is never wanting, and it forms the principal distinction of these 

 shells as compared with those of the Myidje. The cartilage is internal, lying 

 either in special processes or in simple grooves of the hinge, which is occasionally 

 strengthened by a few ribs or laminaj, but very rarely has any additional teeth. 

 The cartilage is, as a rule, bounded internally by a solid ossicle, variable in form 

 and position ; an external ligament is besides often present, but it is generally small 

 and thin ; the muscular impressions are, on account of the thickness of the shell, 

 slightly developed, they are large, and the posterior is usually deeper and more roimded 

 than the anterior ; the pallial impression is also faint, but the sinus usually deep. 



If we summarize the characters of the family, we find that the most important 

 distinctions of the shells are its thin pearly structure and the edentulous hinge. 

 These at least arc almost the only characters upon which we can depend in the 

 determination of fossil species. The shells are generally inequilateral, but some- 

 times the anterior, sometimes the posterior, side is the longer, and the last one 

 usually has a wide gape at the end 



