138 . CRETACEOUS PELECYPODA 



XIII. Family,— FETBICOLIDJE. 



(LITHOPHAGA3 Lamch, et auetorum). 



The animals have a more or less prolonged, thin, sub-cylindrical foot, generally 

 provided with a byssal groove, the byssus itself being, however, sometimes 

 rudimentary, sometimes obsolete ; the mantle margins are united along the greater 

 part of the ventral side ; they are thickened and reflected over the edges ; the 

 siphons are for a shorter or greater distance from the base united, but separated 

 towards the end, and each of the orifices is fringed ; one pair of gills exists on 

 each side ; they vary in size, as do also the palpi, the length and height of which 

 still more depend upon the size of the animal than the corresponding dimensions 

 of the gills. 



The shells are solid, compressed, or more or less inflated, slightly gaping in 

 front and more widely behind ; they usually burrow in stones, very rarely in 

 loose sand; the valves are inequilateral, sometimes unequal, and occasionally 

 covered with a horny epidermis ; hinge small, with two or three cardinal teeth 

 in each valve; no lateral teeth are present; ligament external; pallial sinus 

 deep and always well developed. 



The nature of the habitat of these shells is the cause of considerable variation 

 as regards their shape, so much so that it is often almost impossible to determine 

 its limits among a large number of specimens evidently belonging to the same 

 species. I have already stated my reasons for treating the present family at the 

 head of the whole order (see p. 136). 



The genera referable to it are Fetricolaria, Fetricola, CJioristodon, Saxidomm, 

 Riipellaria (as emended), and Venerupis. In placing the last genus in this family, 

 I follow the classification proposed by Deshayes; and there can be no doubt that 

 in habits, as well as in the organisation of the animals and shells, this classification 

 is preferable to that in the tapesin^ ; I shall again allude to this point when speak- 

 ing of this last sub-family. Eischer (Jour, de Conch., vol. v, 1856, p. 324,) 

 says that Naranio ought to be classed next to Ghoristodon, and H. and A. Adams 

 consider both genera as identical; but I have already remarked (p. 91) that 

 judging from the figure in the Proceedings of the Zool. Soc. for 1863, there appears 

 to be a strong relation between that genus and dementia, as well as the fossil 

 Fsathura of Deshayes. The study of the anatomy of dementia must now show 

 what position in the system it ought to take ; it either belongs, with Naranio 

 and allied forms, to the present family, or to the dosiniin^, or to the familv 

 Glauconomyidje, Weinkauff, (Conch, des Mittelmeeres, vol. i, p. 94,) describes 

 with FetriGola and others also Lucinopsis, Cypricardia, and even Gyamium in one 

 family which he calls Litropraga after Lamarck. 



In a fossil state there are not many species of Fjeitmwolidje known. Some 

 forms, similar to Fetricola, already occur in the trias, a few were described from 

 the jura, but only in the cretaceous deposits do we meet the first satisfactorily 



