44 6 Transformations of Planorbis. [June, 



Once at the Sorbonne in Paris, Professor Hebert, a distinguished 

 French palaeontologist, showed me a magnificent series of un- 

 coiled Cretaceous Ammonites, and by way of testing these con- 

 clusions, I asked this question : 



" Where, M. Hebert, is the closely coiled symmetrical form 

 which ought to be found with these ?" He turned to the other 

 side of the room, and pointed out the required form, saying, 

 " There it is, I found it last summer." 



These and other facts of a similar kind indicate that when 

 physical surroundings become unfavorable to any organization, it 

 takes on a certain series of retrograde transformations. 



When they occur in the individual in the decline of life, or 

 prematurely in the course of growth, through disease, they are 

 similar to the characteristics of whole species, and even groups 

 of degenerate forms. As in the case described above, Fig. 22, PI. 

 vii, is taken from an aged specimen, but it is distorted in the same 

 way as the diseased specimen, Fig. 21, PI. vii, and the retrogress- 

 ive or degraded species of Series 1, PI. vi. 



All the facts corroborating this assertion have already been 

 published and are too numerous to be described in the limits of 

 a paper like this. 



The direction of the spiral is backwards or away from the 

 mouth in the Gasteropods and towards the mouth in the Lamel- 

 libranchs ; in, both, however, it is in the direction in which gravi- 

 tation acts with greatest effect. 



The Lamellibranchs have a split shell, with two valves. Each 

 valve is unsymmetrically spiral, b.ut there is one on each side of 

 the vertical axis or axis of gravity, so that they balance each other 

 and together form a bilaterally symmetrical shell, Figs. 15, .18, 

 seen from the side, and 16, 17, from behind; a, spiral of right 

 valve, and b, spiral of left valve. 



The spiral and the outlines of the 

 side except in those forms which chan 

 habitually on one side. 



ual on « 



These ar 

 ance with 



edi 



the 



:formed and unequal 

 amount of support 



; the deformation is 



and the resistance 



of the sur- 



roundings. 



Thus oysters m 

 fitting the curvatu 



dv grow t 



the ri 

 ice, Fig 



ght or 1 

 .13, PL 



eft or very i 

 vii. In the: 



rregularly, 

 /oungthey 



