18 CRETACEOUS PELECYPODA 



and their number sometimes changes at different stages of growth in one and the 

 same species. 



Omitting the indistinct traces in palajozoic formations, true rnoLADiy.E are with 

 sufficient certainty known from the Jurassic period, and during the cretaceous time 

 they were ah-eady tolerably numerous, though unfortunately often not so perfect as 

 would be requu'ed for a correct generic determination. The tertiary period supplies 

 a large number of characteristic forms ; these arc in general very much like the 

 recent ones. 



The FHOLAViNJE borc in wood, all kinds of rock, sand, mud, and in other shells, 

 or corals ; their borings are not usually very deep, and the hollows only occasionally 

 entirely or partially lined with a shelly tube. In the fossU Teredina the tube is grown 

 together with the valves, as in the Gastrocbmnibm. Tryon (Proceed. Acad. Nat. 

 Sc, Phil., 18G2), in retaining the puoladiNjE as a family, proposes to separate it into 

 true PHOLADiN^E and jouannetinjE, the former having the anterior hiatus always 

 open, the latter closed when adult. In some respects the division may be deemed 

 convenient, but it cannot be considered as of any great importance, because the case 

 generally depends merely upon the stage of growth whether the animal closes its 

 shell in front or not. 



The following is a summary of the genera as at present known. I shall enu- 

 merate them according to their relations to the teredinin.k, shoAving gradually, 

 by a decrease of the accessory valves, the general form of their shells, &c., a passage 

 to the Gastroch^nidje. 



1. Teredina, Desh., 1824. Valves Teredo -like, but firmly connected with the 

 tube, one small accessory valve covering the umbones and extending anteriorly, 

 another large accessory valve occupying the whole of the front of the valves and 

 extending below. Dcshayes in his last edition of the Paris fossils, (18G0, vol. i, 

 p. 124, etc.,) described under this genus three species, T. personata, Lamk., 

 T. Oweni and Heberti, Desh. 



The presence of accessory palettes designate this genus as one of the pholadik^, 

 especially as no siphonal palettes seem to exist, although the usual bifid terminations 

 of the tube are veiy much like those of Ktqjlms, and are no doubt an important 

 character of the teredinin^. It would probably be best to constitute a special 

 sub-famUy for this genus, but it will only have its full value when the existence 

 or non-existence of palettes is sufficiently ascertained. The termination 

 of a tube figured by Romer under the name of T. dentattts, (Xord-deutsch. 

 Kreidegeb., 1841, pi. 10, fig. 9,) is, as abeady pointed out by Pictet (Pal. 

 Suisse, 3"' Ser., 3"'' part., p. 23, 18G4), quite as likely a Teredina. Romer's 

 Teredina clavata (ibid, pi. 10, fig. 10), from the S^nonien near Quedlinburg, 

 the same author, as well as Geinitz and others, believe to belong to Clacagella, 

 or to an allied genus. The specimen described and figured is, however, quite 

 insufficient to wan-ant an idea as to the exact generic characters of the fossil. 



Romer's Fistulana constricta (ibid, p. 76, pi. 10, fig. 2), which he identifies 

 with Phillip's riiol. (Marksia ?) conslricta, (Yorksh., jjt. i, pi. 2, fig. 17,) and which 



