OF SOUTHERN INDIA. 307 



1. ^theria. Lam., 1808. Shell higher than hroad, attached with the lower 

 valve near the beak, upper valve somewhat smaller ; ligament tortuous, partially 

 stuck in a groove of the areal margin of the left or attached valve; muscular 

 impressions two, wide apart, the anterior very long and irregular ; type, JE. semi- 

 lunata. Lam., which is, as likewise the other two or three known species, from 

 Africa. 



2. Millleria, Fer., 1823. Shell free, inequilateral, equivalve and thin, when 

 young, witli a thin ligament and two muscular scars ; when adult, the beaks of the 

 lower, partially attached, valve become considerably produced, while the upper or 

 right valve remains smaller ; the anterior muscular scar becomes obsolete and the 

 pallia! impression quite indistinct; type, 11. lobata, Fer., is the only known species 

 from South America. 



3. Bartlettia, H. Ad., 1866, (Froc. Zool. Soc, Lond., 444). Shell free, 

 equivalve, closed, inequilateral, the anterior portion being peculiarly produced and 

 rugose, the ventral edge insinuated, hinge edentulous ; ligament marginal, partially 

 internal, supported by strong fulcra, muscular scars two, marginal; pallia! line 

 entire; type, Bart. Stefanensis, Moricand, from the Amazon river, originally des- 

 cribed as JEtheria. 



VIII. Order,— ARCACEA. 



The families belonging to this order are four, Trigoniidje, Nuculanidm, 

 NucuLiDjE and Arcid^. They represent, though not without exception, the type 

 of dimyariau Pelecypoda without any special in- or ex-halent siplions. The 

 NucuLAXiD^ appear to be the most highly organized, possessing two more or less 

 prolonged and united siphons ; the animals of all the other families have, however, 

 no trace of siphons, the mantle margins being open all round ; there are always two 

 retractile muscles present, a pair of gills and a pair of palpi on eaoli side. In the 

 TsiGONiiD^ we find for the first time the existence of certain ocelli at the edge 

 of the mantle, such as we see greatly developed in the Pectinibx. 



The shells have one character in common, consisting in the numerous denti- 

 culations of the hinge-teeth. The TRiaosiiD/E indicate a great relation to the 

 Crassatellib^, possessing hinge-teeth somewhat similarly arranged as in these. 

 The older forms fSchhodits and MyophoriaJ have them almost entire, like other 

 AsTARTiB^, but Tngonia and its allies have them striated or denticulated. 

 The three other families agree in the numerous denticulations of the cardinal 

 margins. I do not think that these denticulations or plications can, strictly speak- 

 ing, be compared with the crenated or denticulated hinge-teeth of Spisula or any 

 other genus of the Pelecypoda. For in these the hinge-teeth actually fit beside or 

 into each other, as they also do in Trigonia, one tooth being in this genus single, 

 the other bifid. In the Nuculanidje, Nuculib^e, and Arcib^ the hinge margins, 

 or, as Gray calls them, hinge-teeth, fit exactly one on or against the other by a 

 great number of small teeth or folds. This character is peculiar to the three 

 mentioned families, and we only find it again partially indicated and somewhat 

 modified in the Aviculibje. 



4 F 



