OF SOUTHERN INDIA. 49 



IV. Family,— MA CTBIBJE. 



As far as the animals of the species, belonging to this family, have been 

 examined, they all have been shown to possess united siphons and a more or less 

 prolonged, laterally compressed and pointed foot ; the palpi, or labial appendages, 

 are four in number, long, narrow and pointed ; the gills are not prolonged in the 

 siphons ; the mantle mai'gins are widely open in front and narrowly behind, some- 

 times serrated at the free edges. 



The shells are, as a rule, equivalve and of moderate thickness ; the hinge has a 

 large cartilage-pit in each valve, one triangular or prominent short tooth in front of 

 it and a much thinner and generally more elongated one posterior to it ; the lateral 

 teeth when present are simple in the left, and double in the right, valve ; the pallial 

 impression is variable, rarely entire, but generally distinctly sinuated posteriorly. 



Though the shells of the Mactridje exhibit a large amount of variation, they 

 all evidently belong to one and the same type, the cartilage-pit and the anterior 

 short triangular- shaped tooth being foimd in all of them, only exceptionally they 

 are not so well developed as seen in the typical species of Ilactra and others. 



At the same time two somewhat different groups, or sub-families, can certainly 

 be conveniently separated, as pointed out by H. and A. Adams, Gray, Conrad, and 

 others. The one has for type the genus Lntraria, the second Ilactra. In the 

 former, the lutuauunm, the shells are usually elongated, the lateral teeth of the 

 hinge are either obsolete, or very small, and the animals have long united siphons 

 more or less protected with an extension of the epidermis which covers the entire 

 shell. This sub-family exhibits in its typical forms, as for instance Tresus and 

 Lutniria, the greatest resemblance to Mya, with which it has been, and is often now 

 (Homer) classed in one and the same family. The animals and shells are exter- 

 nally almost identical in both. For these reasons I prefer, therefore, to class the 

 whole of the Mactridje next to the Myid^, instead of following H. and A. Adams 

 who place them in the VENEBACEA. 



The 3iactriNjE generally have a more equilateral, sub-triangular shell, the hinge 

 with the lateral teeth well developed, and the animals possess short siphons covered 

 only at their base with the epidermis of the shell. Thus, as compared with the 

 MriD^, the present two sub-families, mactrin^ and lutrariinm, may be said to be 

 analogous to corbvlinm and myin^, the divisions being based respectively on 

 perfectly similar characters. 



A list of the recent species of the Mactridje has been published by Mr. 

 T. A. Conrad in pt. 3, vol. iii of the Amer. Journal of Conchology. 



I might also have placed near this family the Crassatellid^,-^^ least the 

 genus Crassatella- but their animals are so closely allied to those of Astarte, that the 

 presence of an internal cartilage cannot be accepted as a character, decisive in point 

 of classification, entirely superseding all others. 



Considering paloeontological evidence the Mactridje would appear to take 

 a high place in the system, and the division of the two sub-families also appears to 



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