10 CRETACEOUS PELECYPODA 



tholabinm the boring instruments are the numerous very fine imbrications of the 

 shell, which very probably contain a number of silicious particles ; in the teujedinin^ 

 such silicious particles have been shown to occur plentifully imbedded in the super- 

 ficial muscular tissue of the front part of the body. The turnings of the shells are 

 effected by the foot, which by its shortness and strong muscular structure is well 

 adapted for that purpose. The idea of the secretion of some kind of acid, which 

 alone would produce such excavations in wood and hard rock, has been justly rejected 

 altogether ; at the same time it appears very likely that the organic fluid secreted by 

 the body,-being at a certain degree of higher temperature, so to say, infiltrated or 

 injected with force by the animal into the finest pores of the material attacked,- 

 must greatly facilitate the boring, inasmuch as it softens the finest particles of the 

 substance all round. 



Species of Pholads have been described already from paleozoic rocks, but they 

 are not sufiiciently characterized. Pieces of wood bored by them, (and probably 

 also by teredinin^e)^, occur in the carboniferous strata. Prom the Jurassic beds 

 several speciesf from Prance, England, and Pussia are well known, although their 

 generic determinations are not always sufficiently certain. The cretaceous period 

 contains, however, a number of good types, exhibiting in general a great similarity 

 to recent forms. We may say that there are about 40 species known to occur in 

 cretaceous rocks; the somewhat larger number belongs to the fholabin^, the 

 smaller to the teredinin^. In the tertiary strata species of Tkoladibm occur 

 sparingly, at least the number of them upon record is scarcely larger than that from 

 cretaceous beds. At the present time their geographical distribution is world-wide, 

 and many of the species appear to possess a greater faculty of accommodating them- 

 selves to different climatological provinces than other molluscs. The geological 

 history of the family distinctly shows that it has its maximum of development in the 

 present epoch. The great similarity in the general structure of the Pholads seems to 

 have been rather an obstacle towards their detailed knowledge, and there can be little 

 doubt that we may soon see the number of species doubled, when conchologists get 

 more accustomed to see the usefulness of a chisel and hammer, or of a hatchet, as 

 welcome companions on their excursions. Along the shores of the Bay of Bengal 

 I have met with several as yet undescribed species. 



Among the most recent and best researches on the Trolabibm are those 

 of G. W. Try on, Junr., who published a monograph of that family in the 

 Proceedings of the Academy of Natural Sciences, Philadelphia, for 1862, and 

 lately, 1867, a revised catalogue of the same in the American Journal of Con- 

 chology, vol. iii, pt. 3. In this last paper Tryon accepts for our shells two 

 family divisions, Froladidje and Terebidm (TeredinidjsJ. The advantage of such 

 a thorough separation appears to be very doubtful, nor does there seem to be suffi- 

 cient ground for it in the organisation, or in the habits of the animals or shells. The 

 character of the organisation is throughout the family the same : a wormlike body 



^ Teredo antiqua, M'Coy, from the carboniferous strata, and others. 



t See Morris and Lycett : MoUusca of the Great Oolite,— Buvignier: Statistique de la Meuse. 



