10 CEETACEOUS PELECYPODA 



FKOLADiNJE the boi'mg instruments are the numerous very fine imbrications of the 

 shell, which very probably contain a number of silicious particles ; in tlie teredininm 

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

 ficial mviscular 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 structm'e 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 palaeozoic rocks, but they 

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

 also by tetibdininm)* , occur in the carboniferous strata. Erom the jvu'assic beds 

 several speciesf from Erance, England, and Russia 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 occiu- in 

 cretaceous rocks ; the somewhat larger number belongs to the PROLADisyE, the 

 smaller to the terebinin^e. In the tertiary strata sjiecies of Fholadidje occiir 

 sparingly, at least the number of them upon record is scarcely larger than that fi-om 

 cretaceous beds. At the present time their geograjihical distribution is world-"ndde, 

 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 maxunimi 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 theu' 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 Pholadid^ are those 

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

 Proceedings of the Academy of Natiu'al Sciences, Philadelphia, for 1862, and 

 lately, 1867, a revised catalogvie of the same in the American Journal of Cou- 

 chology, vol. iii, pt. 3. In this last jiaper Tryon accepts for om- shells two 

 family divisions, Proladid^ and Teredibm (TEREDimBM) . 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 L3-cett: MoUusca of the Great Oolite, — Buvignior: Statistiiiuo de l;i Mouse. 



