THE NAITIMS. 43 



As to the ])oints of difference between u.s, I would .say that I have 

 'examined hundreds of Valvata sineera in all its varieties, and am 

 certain that V. striata and " V. mergella" are nijthing but extreme 

 forms, which imperceptibly merge into the sineera. This is shown 

 by numerous British American and U. S. s])ecimens. Lyogyrns 

 ■lehnerti is a sinistral monstrosity, no more entitled to specific rank 

 .and name than the sinistral specimens occa.sionally found in every 

 species of Campeloma (Paludina). 



Both Thompso7iia and Thovisonia 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 spinosiis. The shell really belongs to Stimj)son's 

 genus Potamopyrgus, as the writer pointed out a few months after the 

 original publication. Potamopjyrgiis is largely represented in New 

 Zealand, Au.stralia and Tasmania, and also in the AVest Indies and 

 ■adjacent mainland of Mexico, Central and South America. Whei'- 

 -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 

 '{Cllthon). They may be either carinated above the periphery, the 

 carina armed with a corona of spines, or else rounded, the supei'ior 

 aspect of the whorls completely smooth, rather flattened, and but 

 slightly convex. In the case of P. spinosiis 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 (3Iex. Land- u. Sussivasser ConchyL, pi. v, 

 figs. 34, 34a) under the name of " Hydrobia eoronata, Pfr." There 

 are some differences between the Continental and Culiau forms, but 

 ■all will probably prove identical. Von Martens having already 

 united all of those knoAvn 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, AV. Africa, 

 and of fossil forms of the same spiny type in S. European Tertiary 

 strata shows that the group is ancient and wide-spread. ]\Ir. Ancey 

 <Bull. Soc. Mai. France, 1888, p. 185) has lately published an Etude 

 3lonographique sur Pyrgulopsis, in 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 



