OP SOUTIIEP.N INDIA. 243 



VI. Order,— LUCINACEA. 



Like every other group of animals which holds a certain intermediate position 

 between a number of distinct types this order includes rather a varied assemblage 

 of forms which it is very diflicult to characterize in a few words. The families 

 belonging to the order are the Lucinid^, Ungulinidm, Urtcinid^, Galieomuibm, 

 SoLjEMTiDM, AsTARTiBm, and Crassatellid^ ; they follow each other accordino^ 

 to their relative relations, as far as it is possible to give an expression to this idea. 

 The typical forms have the mantle margins disunited, and a single siphonal opening 

 posteriorly, sometimes prolonged into a tube, but there never are two equally 

 prolonged siphons present. However, many of the Lucinid^ possess two siphonal 

 openings, the inhalant being sometimes produced ; all the others have only one open- 

 ing, except the Crassatellid^, which appear to have none, though its place is indi- 

 cated by a very thin commissure in the mantle, they thus form a transition to the 

 next order. Again, the Galeommibm and Solemyibm have the inner mantle margins 

 partially united, and recall the structure of some of the MYACJEA. The form of 

 the foot is variable, either vermiform and cylindrical, or angularly bent, as in some 

 Gardia, or compressed, as in the VUNJERACJEA. There is generally a pair of gills 

 on each side of the body and one pair of palpi ; rarely the latter become nearly 

 obsolete. 



Although some of the genera combine in their organisation characters which 

 they have in common with higher organised Pelecypoda, the greatest number show 

 a lower organised type, than that of any of the previous orders. The LTIGI- 

 JSfAGUA must be considered as the type of Pelecypoda provided with only an. 

 exhalant opening, while the admission of water to the gills can take place in the 

 entire extent of the disunited mantle margins. Somewhat different would be our 

 conclusions had we to go simply by the characters of the shells in classifying them, 

 for the greater number of them is closely allied to the VBNERA GBA, possessing 

 a small number of hinge-teeth which fit alternately in the two valves ; there are 

 two muscular scars, and the pallial line is always entire. In the characters 

 relating to the rounded or oval form, and the presence of cardinal and lateral 

 hinge-teeth of the shell, the Lvcinibm, (and particularly the corbinmJ, show a 

 very great relation to the Carbiibm ; the JErtcinib^ and Galeommibm recall 

 the Myibm and Anatinibm, possessing a thin shell with few hinge-teeth and 

 an internal cartilage ; the Solemyibje have a shell which differs from that of Cyrto- 

 daria almost only by its thin structure, its general form being also that of many 

 SoLENiBj^.; the Astartibm have their nearest allies partially in the Venerium, 

 partially in the Gyfrinibm, both in the form of the shell and the distribution of 

 the hinge-teeth; and, lastly, the Grassatellibm strongly recall the Mactribm. 

 This association of shells so varied clearly indicates that further research has 

 yet many gaps to fill up in the way of connecting links from one type to the other. 

 I will only briefly allude to the geological history of the order. The 

 SoLEMYiBM appear to be the oldest of all ; they certainly occur already in the 



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