320 ROBERT TRACY JACKSON ON THE 



be carried fui'ther and for such inqiiii-j we would refer to published works on couehology 

 and palajontology. Believing that the iuequivalvulnrity of the above genera is due to 

 the conditions of environment acting on a bivalve shell, I would refer to species of at- 

 tached Brachiopods in which is found fuither proof of the correctness of the conclusion. 

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

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

 valve, which is rugose and concave, is strikingly difterent from the ui)per valve which is 

 flattish and smooth. The same feature is shown in sjjccies of StrojjJialosia receutly dis- 

 covered and shown me by Di'. Beecher. Productus proioscidcus of the Carboniferous 

 has a tubular highly modified attached valve aud a nearly normal free valve'. 



In the various genera considered, the free valve is that which is least modified and 

 nearest resembles the form existing in ioth valves of less aberrant related genera. It is the 

 attached, cemented valve, which is highly modified and so much so as by itself in some 

 cases 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 siibnacreous portion which are filled with a soft, white, chalky deposit. These cav- 

 ities are also noticeable in many fossil oysters and Giy[)hjeas. In Exogyras they are com- 

 monly very large and abundant, especially near the umbos where shell growth takes 

 place rapidly and to a great thickness. Exoggra costata shows these cavities well, and 

 the}^ are highly developed in Exogyra aquila, Goldf. and E. couloni, d'Orb. from the 

 Cretaceous 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 lashion 

 ai'e frequent, although this shell never builds ti'ue eamerated structui'e. 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 Woodwai'd (77) compared the structure of the oyster also with 

 eamerated structure in species of Teredo and Caprinella which he figures. This last 

 genus is remarkable for the legularity and size of its chambers which truly I'esemble 

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

 other bivalves, which have eamerated 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 sal'ely reversed and we may say, 

 that bivalves whicJi are cemented by one vcdve to extraneous bodies liave a tendency to pro- 

 duce a eamerated structure of tJie 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 general, whereas no free lococomotive forms of Pelecypods, however 

 thick the shells, have been found to possess such structure.* 



'In the ik'velopiiijj shell of Vermehis radirula. fig. ',), 'It is important to notice that fossil corals of the Tet- 



p. 204, I have observed in well-preserved specimens that racoroUa type have a highly developed eamerated strnct- 



the shell is evenly spiral and regular groAving while young nre. So similar is their ajtpearance in their regular eham- 



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



regular loosely spiral features characteristic of tlie adult Cephalopods especially of the Cyrtoceras type which tliey 



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



growth. striking resemblance both in external aud internal form to 



