THE NAUTILUS. 43 
As to the points of difference between us, I would say that I have 
examined hundreds of Valvata sincera in all its varieties, and am 
certain that V. striata and “ V. mergél/a” are nothing but extreme 
forms, which imperceptibly merge into the sincera. This is shown 
hy numerous British American and U.S. specimens. Lyogyrus 
Jehnerti is a sinistral monstrosity, no more entitled to specific rank 
and name than the sinistral specimens occasionally found in every 
species of Campeloma (Paludina). 
Both Thompsonia and Thomsonia are preoccupied as generic names 
in Zoology. 
I take this occasion to correct a mistake of my own which 
apparently has mislead Mr. Ancey. Several years ago Prof. R. E. 
‘Call and the writer described a species of spiny ‘rissoid from Texas 
as Pyrgulopsis spinosus. The shell really belongs to Stimpson’s 
genus Potumopyrgus, as the writer pointed out a few months after the 
original publication. Fotamopyrgus is largely represented in New 
Zealand, Australia and Tasmania, and also in the West Indies and 
adjacent mainland of Mexico, Central and South America. Wher- 
ever they are found, the species are nearly all subject to a dimorphism 
even more puzzling at first than that of the spiny forms of Neritina 
(Clithon). They may be either carinated above the periphery, the 
carina armed with a corona of spines, or else rounded, the superior 
aspect ot the whorls completely smooth, rather flattened, and but 
slightly convex. In the case of P. spinosus C. and P., I have called 
the smooth form “ Hydrobia texana,” at that time not knowing the 
mutations to which these forms were subject. The P. spinosus has 
been figured by Strebel (Mex. Land-u. Siisswasser Conchyl., pl. v, 
figs. 54, 34a) under the name of “ Hydrobia coronata, Pfr.” There 
‘are some differences between the Continental and Cuban forms, but 
‘all will probably prove identical, Von Martens having already 
united all of those known to him from the Americas, under the old 
name of coronatus, Pfr. The American species agree with the 
Australasian in the dentition, which is quite distinct from that of 
other rissoid forms. The presence of a species in Liberia, W. Africa, 
and of fossil forms of the same spiny type in 8. European Tertiary 
strata shows that the group is ancient and wide-spread. Mr. Ancey 
(Bull. Soe. Mal. France, 1888, p. 185) has lately published an Etude 
Monographique sur Pyrgulopsis, 1 which he has included the 
American forms of Potamopyrgus known to him (but not one-half 
‘of the so-called species in our literature) in a section of Pyrgulopsis 
