OF SOUTHERN INDIxi. 139 



determined species. They gradually increase in the tertiaries and attain their 

 maximum of development in the present seas. About 70 species are known 

 recent, and when the rocks and coral reefs, as, for instance, those of our eastern 

 seas, have been more assiduously searched large additions may be expected. Their 

 distribution is yery large ; with the exception of the arctic seas, they occur almost 

 everywhere, especially, however, where coral reefs are to be found. 



1. Fetricola, Lamarck, 1801. Shell oviform, or sub-cylindrical, produced 

 posteriorly and more widely gaping than anteriorly, sometimes with unequal valves, 

 the right one being larger and reaching with the edges over the left one all round ; 

 surface radiately and concentrically finely ribbed, and often lamellar, usually covered 

 with a thickened epidermis ; hinge with two cardinal teeth in each valve ; some- 

 times the posterior cardinal of the left, or the anterior of the right valve become 

 obsolete; or in very aged specimens all the hinge-teeth disappear, as they do, for 

 instance, in Saxicava ; again, in others there is in the left valve a distinct indication 

 of a third posterior tooth, and an additional anterior tooth in the right valve is also 

 indicated. 



There are two different types of shells classed under the genus Fetricola. 

 The one,— and for these the name Fetricola ought to be reserved,— are represented 

 by P. lithophaga, Eetz.; they have an oval shape, are thick anteriorly, attenuated 

 posteriorly, more or less globose, solid, with the hinge-teeth usually short and thick. 

 The pallial sinus varies, being either wide and obtuse (as in the type species) or 

 angular (as in P. monstrosaj , 



The miocene shell which Conrad (Proc. Phil. Acad., 1862, p. 576,) introduces 

 as Fliorytis centenaria (Fetricola, olim) in the family Fbtricolidm is to all appear- 

 ance a Capsa (Asaphis), 



la, Fetricolaria, Stol., 1870. The second group is represented by Fet. phola- 

 diformis. In this the shell is very much elongated, sub-cylindrical, the sinus narrow 

 and very deep ; the hinge has two teeth in each valve, attached below the hinge 

 area and curving upwards ; in the right valve the anterior tooth is hook-like, the 

 posterior much larger, broadly laminar and bipartite ; in the left the anterior is 

 very large and bipartite, its anterior portion almost representing a separate hook- 

 like tooth corresponding to the anterior tooth of the right valve, the posterior portion 

 is thick and prominent and longitudinally grooved ; besides this there is a small, 

 sometimes obsolete posterior cardinal tooth. Although there exists a decided 

 similarity between the hinge of Fetricola and Fetricolaria, there is a very marked 

 distinction between them. Fischer examined the animal of Fet, denticulata 

 Hanley, and he records (Jour, de Conch., vol. v, p. 326,) several important distinc- 

 tions in the elongated form of the palps, gills, &c., as compared with those of Fetricola 

 proper, but he is inclined (ibid p. 329) to consider these as variations of one and 

 the same type. No doubt such is and must be the case in all other families also 

 but at the same time the distinctions pointed out appear to me to be in favor of a 

 separate (at least sub-generic) grouping of these forms. I may add that there is 



