OP SOUTHERX INDIA. 453 



The OsTREiD^ are principally distiuguislied from other allied forms of mono- 

 myarian Pelecypoda by their single gills and almost entire absence of the foot. It 

 is, however, not certain that this latter character would apply to all the forms refer- 

 able to the family, as, for instance, to many of the fossil Gruphcsce which have no 

 trace of attachment. And as in young oysters there is a small indication of the foot, 

 this organ did very probably develop further in those species which remained free, 

 while it became obsolete in others which selected a sessile habitat. Many concholo- 

 gists have lately inclined to uniting all the different forms, which had been classified 

 under the three principal genera Ostrea, Gryphcea and Exocjyra, under the first-men- 

 tioned name. I have here retained the old suggested tri-division and also noted some 

 of the sub-genera, because I fully believe that the three above mentioned genera are 

 as well founded as those of most other Pelecypoda, the species grouping themselves 

 round certain generic types. Naturally, when we become acquainted with a large 

 number of closely allied fossils which have descended from one common stock, we 

 find that the many intermediate forms greatly interfere with a strictly definable 

 classificatory arrangement. And this is very much the case with fossil Ostreidje ; 

 their remains are among the most common fossils, and their shells are often preserved 

 in beds where, besides FectinidjE and Aviculidje, hardly any other kind of shells 

 remain unaltered. The consequence is, that the generic distinctions must be framed 

 a little wider, and based upon more general characters, than would probably be 

 thought admissible in other families of Pelecypoda. Even those conchologists, 

 however, who advocate the generic unity of Ostvea always admit the convenience 

 of most of the divisions here adopted. 



There have been very few Ostueibm recorded from palaeozoic rocks ; the re- 

 mains are uncertain, though it is not probable that the family is altogether absent 

 in those old formations. In the Trias, both Gryphcea and Exogyva are certainly 

 represented, and probably also Ostrea, In the Lias the species of the two first 

 named genera are found in abundance, and those of the third become well marked. 

 In the Dogger and Malm the number of species has considerably increased ; in 

 the cretaceous period it is again larger and attains probably its maximum. The 

 genera are there slightly differently represented, viz., the number of species of 

 Gryphcea diminishes as compared with the Jura ; Kvogyra reaches its maximum ; 

 and Ostrea presents the greatest variety of form, but the number of species them- 

 selves is not very large, they chiefly belong to the sub-genus Alectryonia. The 

 tertiary epoch was mostly inhabited by Ostrece, the two other genera are compara- 

 tively very scarce ; the total number of the species of this family is most probably 

 smaller than that in the cretaceous period alone. On the whole, there are now 

 about 600 (more or less correctly defined) species of fossil Ostrea; known, while 

 that of the recent species does not nearly amount to one-sixth of that number. 



1. Ostrea, Linne, 1758. Shell irregular, inequivalve, attached with one valve, 

 the other being usually smaller, more or less inequilateral, with straight or slightly 

 curved beaks ; ligament lodged in a median groove below the beaks, the ligamental 

 area more or less extending on each side of the groove; it is marked with 



5 T 



