384 TAYLOR : REMARKABLE ATAVIC SPECIMENS. 
The sinistral Planorbis spirorbis found by Mr. Wood, was 
obtained early in December, 1896, from the overflow stream of 
the mill pool at Hill Hook, Sutton Coldfield. 
This stream is very clear, sustaining a rich vege- 
tation, chiefly water-starwort and watercress, while 
the bed of the stream is thickly strewn with valves 
of Pisidia and Sph. corneum, var. flavescens. 
A careful study of this sinistral shell 
Atavic Sinistral shows that it is not simply a reversed cork- 
monstrosity of 
Planortis spirorbis’ screw monstrosity of the ordinary dextral form, 
Hill Fook Mill with a dextral animal for its occupant, but a 
Stream, 
Sutton Coldfield. — syecimen whose peculiarities have arisen by a 
reversal of the hyperstrophic process, and which therefore repre- 
sents the original orthostrophic form of the species, the animal be- 
ing a sinistrally organized animal within a sinistrally coiled shell. 
The dextral turreted specimen was obtained from the 
waters of an old marl pit, at Hill Hook, about the same time 
as the sinistral specimen, and is a much more 
interesting and remarkable shell. 
To a superficial observer, the shell might 
be supposed to be merely a turreted mon- 
strosity of the ordinary dextral form of the 
species, but like the sinistral specimen, this is 
also shown on critical examination to owe its 
: Care o 2 5 Atavic Dextral 
chief peculiarity to this reversion to the ancient _ monstrosity of 
t the primitively sinistral coiling having lea gaa 
@ ie B iling havin iill. x 6, 
YP& P y 8 5 Mar!Pit, Hill Hook, 
become reversed, as the shell has undoubtedly Sutton Coldfield. 
been formed by a dextrally organized animal, which has thus 
a shell coiled in harmony with the structure of the animal, proy- 
ing it to be a dextral monstrosity of the atavic sinistral form. 
In my discussion of this subject (4c, p. 112) I pointed 
out that of the four theoretically possible modes of convolution, 
only three were at that time known to exist, but this blank in 
our knowledge is now filled up by the discovery of this atavis- 
tically dextral form, which is, so far as I know, quite unique. 
J.C., vili., Apr. 1897 
