18 CEETACEOUS PELECYPODA 



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

 same species. 



Omitting the indistinct traces in palaeozoic formations, true feoladinje are with 

 suflB.cient 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 required for a correct generic determination. The tertiary period supplies 

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

 recent ones. 



The PROLADiNJE bore 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 fossil Teredina the tube is grown 

 together with the valves, as in the Gastrochmnidm. Try on (Proceed. Acad. Nat. 

 Sc, Phil., 1862), in retaining the prolabin^e as a family, proposes to separate it into 

 true PHOLADiNJE and jouann^jtina^, 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 ihe genera as at present known. I shall enu- 

 merate them according to their relations to the tebedininj^, showing gradually, 

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

 to the Ga8troch.enid^, 



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. Deshayes in his last edition of the Paris fossils, (1860, vol. i, 

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

 T. Oiveni and Seberti, Desli. 



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

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

 of the tube are very much like those of Kuphtis, and are no doubt an important 

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

 sub-family 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 Homer under the name of T. dentatus, (Nord-deutsch. 

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

 Suisse, 3'''' Ser., 3"^' part., p. 23, 1864), quite as likely a Teredina. Romer's 

 Teredina clavata (ibid, pi. 10, fig. 10), from the Senonien near Quedlinburg, 

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

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

 insufficient to warrant an idea as to the exact generic characters of the fossil. 



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

 with Phillip's PJioL (Martesia ?) constricta, (Yorksh., pt.i,pl. 2, fig. 17,) and which 



