382 JOURNAL OF CONCHOLOGY. 
ON TWO REMARKABLE ATAVIC SPECIMENS 
OF PLANORBIS SPIRORBIS Mull. | 
By JOHN W. TAYLOR, F.L.S. 
(Read before the Leeds Branch of the Conchological Society, March zoth, 1897). _ 
Mr. ALBERT Woop, of Sutton Coldfield, a most enthusiastic 
and successful conchological student, who is systematically 
working out the molluscan fauna of his neighbourhood with a 
view to the eventual preparation of an exhaustive monographic 
account of the mollusca of the Park, and who, in furtherance 
of this object, has recently issued for private circulation an 
interesting epitome of the results he has attained up to the 
present time, has been fortunate enough to find almost simul- 
taneously the two remarkable shells of lanorbis spirorbis, 
which form the subject of the present note, and are prob- 
ably by far the most interesting specimens that have up to 
the present time rewarded his investigations. 
These specimens are of great interest, one being sinistrally 
and the other dextrally coiled; but a careful study of their 
peculiarities shows both shells to be atavistic or reversions to 
the ancestral or primitive type of the species, as in this and 
similar carinate species we are able by a critical examination of 
the shell to confidently proclaim the mode of organization of 
the animal forming it, and by applying our knowledge to these 
two special cases we see the sinistra] specimen to be a reversion 
to the original coiling natural to the species and in accord with 
its sinistrally organized inhabitant, while the dextral shell is a 
reversed monstrosity of the orthostrophically sinistral form. 
; For those who have not studied the subject it may be 
necessary to state that the //anordes are sinistrally organized 
animals, primitively possessed of sinistrally coiled shells, but 
by the peculiar modification or specialization, known as hyper- 
strophy, the shell has become a dextral one, this change, how- 
ever, not affecting the sinistral organization of the animal. 
J.C., viil., Apr. 1897. 
