112 
MONSTROSITIES — nyPERSTROPHY. 
(if its viscera, lint tlie basal part of flic sliell liecomes li}" transposition 
of liy})erstroplnc orowtli the upper or spire face ; in tlie latter case, 
it is tlie or^-ans of the animal that lieconie actually transposed in 
jiosition, those niion the ri,nht side become idaced on the left, and 
I'la- wrsd, but tlie shell is simply reversed in its direction of convolu- 
tion, the basal and upper iiices of the shell retaining their positions 
as in the normal individuals, and are not transposed as in hyjier- 
strojihic specimens. 
'I'lieoretically, there may thus be tour dittereut jdiases in the coiling 
of Planorboid shells. Viewing the shell of Phiunrltis as being now 
dextral, they may be simply sinistra! or simply de.xtral, or siinstral 
or dextral by atavism, altbougli only three of these phases are actually 
known, the simply dextral and simjily siinstral forms, and the 
atavistically siuistral. 
I )iaccrnmmaiic li)iurc'; showing the probably’ actual motlc by’ wliich the change from a sinislral 
orihostrophic slicll to a hyperstrophic and psemlo-tlextral one lias taken place. The lieart is seen 
to apjiarcnily move from the base to the upper part of the whorl. 
In the series of figures preceding, 1 have endeavoured to dejiict 
diugrammatically the gradual and actual mode by which the jiseudo- 
de.xlral tor ultra-sinistral, as they are sometimes though less happily 
t('rmed) shells of Pomphnlyx, PhinorhiK, etc., have jirobably arisen. 
'I'be tig. 248 is assumed to represent an elongated Plnjtfa carrying 
its shell with the apex directed to the rear as is normal ; the actual 
and jiarticular mode of carrying the shell by the different sjiecies, is 
doubtless always determined by the mechanical laws governing its 
easiest portability in the medium in which they live. Fig. 244 is an 
intermediate or trausitioiial form, which buds its original among some 
of the foreign genera. Figs. 24;) — 247 may all be regarded as more 
or less faithfully re}ireseutiug conit'/is, as the sjiire is still 
slightly exserted on the. left side, and the shell carried nearly upright 
as is usual. Figs. 24S and 24!), which show hyperstrophic develop- 
ment a little further ailvanced, also demonstrate how uiion any 
relaxation of muscular effort by the animal the shell falls naturally 
into a more or less horizontal and truly dextral position. Figs. 2;)() 
and 2;jl show the process advanced a stage further as in Plauorhis 
