( 498 ) 
XXXIX,^ — On a Reversed Species of Fusus, 
(Fusus retroversus.) 
By the Rev. John Fleming, D. D. F. R. S. 
M.W.S. &c. Minister of Flisk. 
{Read 5th April 
It is well known to British conchologists, that sinistral, 
or reversed spiral shells, are of frequent occurrence among 
the terrestrial and fluviatile mollusca, while they are seldom 
to be met with among those which inhabit the sea. Exten- 
sive genera, indeed, occur in the former groups, in which 
all the species exhibit reversed whorls, not as a monstrosity, 
but as a permanent feature. The genera Clausilia and 
Vertigo, in the terrestrial, and Physa, Aplexa, and Plan- 
orbis, among the aquatic pidmonifera, are striking exam- 
ples, and embrace about twenty indigenous species. 
Among the marine spiral shells belonging to the branchi- 
ferous mollusca, those with sinistral whorls are of such rare 
occurrence, that not more than one species in a genus has 
yet been detected. In the British Fauna, only two species 
have hitherto been recorded among the recent kinds, viz. 
