104 HYATT ON THE TERTIARY SPECIES 



alopods, G-asteropods, or Lamellibranchs, and their peculiar shapes, can be accounted for by 

 the different ways in which the attraction of gravitation would act upon the excreting bor- 

 der of the mantle through the weight of the shell itself, or by the natural growth of this 

 part when freed from the weight of the shell. 



Thus the oyster, pecten, etc., show during the adult stages distortions and a peculiar 

 horizontal growth which can only be accounted for by the support they receive, either 

 from permanent attachment, or by resting on one valve. 



The evidence here seems to show that the shell must grow in the direction resulting 

 from the action of the two forces, the movement and growth of the tissues and the oppos- 

 ing force of gravitation. The extraordinary shapes and combinations of asymmetry and 

 symmetry in different parts of the same animal as exhibited in the mollusca, all seem to be 

 resolved when we can account for the influence of gravitation upon a fixed or moving 

 organism, allowing for the reactions occasioned by growth and heredity. 



APPENDIX III. 



The remarks on p. 76, with regard to PI. pseudotenuis, ave misleading. Since this page 

 was printed I have undertaken with the help of two assistants to revise and re-arrange my 

 collection. This has led to the finding of several specimens of PI. pseudotenuis. These 

 show that PL pseudotenuis is a form which is genetically connected with PI. minutus. It 

 has all the characteristics and peculiar aspect of that species in the young, and is never so 

 stout at any period as PI. Kraussii. The latter has a shell which resembles it in color and 

 general aspect but not in its proportions, and is also approximate to PI. pseudotenuis, very 

 closely in some specimens which have a prominent thick carination. These are very closely 

 similar to PL pseudotenuis, and I think led Dr. Hilgendorf to trace pseudotenuis into 

 Kraussii instead of into PL minutus with which it seems to be connected. There have 

 also been found two specimens of PL pseudotenuis with a sulcation on the upper side 

 of the whorl, which confirms this conclusion ; as any one will see, even from an exam- 

 ination of Hilgendorf's own figure, that such a sulcation would render even the extreme 

 form of PL pseudotenuis very similar to PL triquetrus, which Hilgendorf himself con- 

 siders a member of the minutus series. Var. Kraussii p. 89, fifth paragraph, is PL pseu- 

 dotenuis. One specimen of PL pseudolemds has been found in Formation /, New Pit, 

 one in k, I, East Pit, one in x same Pit, and one in I, Old Pit. 



This revision of the collection has also led to the discovery of several diseased forms of 

 PL trochiformis, which are very interesting. They are dwarfed. The spiral is partly 

 unwound and then closed up again in com-se of growth, but is even then much contracted. 

 In fact a very slight increase in the characteristic tendency of the growth, as shown by 

 our specimen would make a whorl larger, but not very unlike PL deimdatus in general 

 appearance. 



APPENDIX IV. 



In revising the collection my assistants have also succeeded in finding in Formation d e, 

 referred to on page 49, line 18, as containing only " two broken specimens of PL oxysto- 

 mus " and also on p. 93, line 22, two well preserved young specimens and one nearly full 

 grown. 



