TAYLOR: REMARKABLE ATAVIC SPECIMENS. 283 
In the chapter upon Hyperstrophy in my “‘ Monograph ot 
the Land and Freshwater Mollusca of the British Isles,” p. 110 
ef seg., I have fully discussed the various modes of coiling to 
which the shells of //axorbis are liable, and shown that theo- 
retically there are four possible modes of convolution, viz. : 
the simply dextral and simply sinistral coiling and the sinistral 
and dextral convolution due to atavism or reversion. 
Diagrammatic figures showing the probable mode by which a sinistrally organized 
mollusk with a sinistral shell, as the primitive Planorbis, may become possessed of a 
dextrally coiled shell, although the contained animal retains its sinistral organization, 
as is evidenced by the heart persisting in its position on the right side of the 
body, as shown in the figures, and also by the sinistral position of the respiratory and 
other orifices. 
The simply dextral or normal form of the species has 
arisen by hyperstrophic growth from the orthostrophically 
sinistral ancestor, the simply sinistral form being merely a re- 
versed monstrosity of the type, the animal inhabitants being 
organized in a manner contradictory to that of their shells, 
the dextral specimens having sinistrally organized animals, and 
the simply sinistral monstrosities being tenanted by dextral 
animals, this anomaly being owing to the hyperstrophic growth 
which by transposing the relative positions of the apex and base 
of the shells changes the dextral shells to sinistral ones, and 
vice versa, without affecting the organization of the animal. 
The atavic sinistrally coiled shells are reversions to the 
primitive form, and the atavic dextral shells the reversed mon- 
strosity. ‘These forms are due toa reversal of the hyperstrophic 
process and results in the original sinistral shell being reacquired 
and the animal again becoming in harmony with its shell. 
Reversal in the direction of the coiling when not acquired 
by hyperstrophic growth or by atavism, is always accompanied 
by a corresponding reversal of the organization of the animal ; 
the sinistral and dextral monstrosities of various species that 
are met with are usually of this character, 
