320 ROBERT TRACY JACKSON ON THE 



be carried further and for such inquii^ we would refer to published wor]c8 on eonchology 

 and ])ala'ontology. Believing that the ineqiiivalviilarity of the a1)ove genei'a is due to 

 the eondition.s of environment aeting on a bivalve shell, I would refer to species of at- 

 tached Bi'aehiopods in which is fonnd further pi'oof of the correctness of the conclusion. 

 Davidsonella rugosa, PI. xxvi, figs. 14-15, the Cretaceous, is soldered to an object of 

 support by tlie lower valve, and here, as in the above cases of Pelecypods, the attached 

 valve, which is rugose and concave, is strikingly difterent from the upper valve which is 

 flattish and smooth. The same feature is shown in species of Strojjlialo.sia recently dis- 

 covered a]id shown me by Dr. Beecher. Productus proboscideus of the Carboniferous 

 has a tubular highly modified attached valve and a nearly noi-mal free valve'. 



In the various genei'a considered, tlte free valve is fhat tckich is least modified and 

 nearest resemhles tlte form existing in liotli valves of less aberrant related genera. It is tJie 

 attached, cemented vedve, whicJiis higldy modified and so much so as by itself in some 

 eases to be with difficulty recognizable as belonging to the Pelecypods. 



In the shell of Ostrea virginiana, PI. xxvi, fig. 13, there are lenticular cavities in the 

 hard subnacreous portion which ai'e filled with a soft, white, chalky deposit. These cav- 

 ities are also noticeable in many fossil oysteis and Gryphjeas. In Exogyras they are com- 

 monly very large and abundant, especially near the innbos where shell giowth tal<es 

 place rapidly and to a great thickness. lExogyrei costcda shows these cavities well, and 

 they are highly developed in EoLOgyra aqiiila, Goldf. and E. ecvlcni, d'Oib. ficni the 

 Ci'ctaceous of Europe. Not infrequently the cavities in living and fossil shells will be 

 found filled with mud, and in such cases it is evident that the shell roofing was formed 

 to include the mud; being caused by the irritating action of that substance on the over- 

 lying mantle. Specimens of 3Iya arenaria, which have included mud in this fashion 

 aie frequent, although this shell never builds true camcrated structure. The shallow 

 chambered structui-e of the oyster was noticed by Professor Owen (50, 51) and was 

 compared by him to similar conditions in the shell of Spondylus, ^theria and some 

 univalves. Professor Woodwaid (77) compared the structure of the oyster also with 

 camerated structure in species of Teiedo and Capi'inella which he figui-es. This last 

 genus is remarkable for the regulai'ity and size of its chambers which truly I'csemble 

 analogous structures in Cephalopods. Owen (50) notes the fact that Spondylus and 

 other bivalves, which have camerated structure, are cemented to extraneous bodies by the 

 shell. I have not made an extensive study of this subject; but what has been made, 

 leads to the conclusion, that this statement may be safely reversed and we may say, 

 that livcdves icfiich are amented hy onevcdve to extraneous hodies have a tendency to jtro- 

 duce ei ceimer(ded structure of tlie shell which is not found in free forms. Thin shelled, ce- 

 mented Pelecypoda, such as Dimya, quite likely would not show cameration, but amongst 

 other groups it is geneial, whereas no free lococomotive foi'ms of Pelecypods, however 

 thick the shells, have been found to jjossess such structui'e.^ 



'In the developing shell of Vermetus radicula, fig. 9, 'It is important to notice that fossil corals of the Tet- 



p. 2i)4, I have observed in well-preserved specimens that racofoUa type have a highly developed camerated strnct- 



the shell is evenly spiral and regular growing while young ure. So similar is their appearance in their regular cham- 



and unattached. As soon as attached, however, the ir- bering that they may often be comjjared to cliambered 



regular loosely spiral features characteristic of the adult Cephalopods esiiecially of the Cyrtoceras type which they 



are introduced and are retained throughout subsequent simulate by their arcuate form. They also bear often a 



growth. striking resemblance both iu external and internal form to 



