XXXV 



'^ Sp'irodon,'' but the toothed columella is nor even a 

 constant specific character. 



The characters assigned to lo, Pleurocera, Angi- 

 treina, Lithasla, Strej^hobasis, Eurycctilon, Goniohasis, 

 Schizostoma, Meseschiza and Anculosa, are by no 

 means of equal value. I regard the first five as 

 members of the Trypanostomoid section of the 

 family, of which lo is a genus, with Pleiirocera for 

 a subgenus. Lithasla should, perhaps, be considered 

 a subgenus only of Angitrema, which is the highest 

 development of this form, having the thickened 

 columella. 



Strejjhohasis occupies a position between Lithasia 

 and Goniobasls, but I think that it, also, might be 

 considered a subgenus of Angitrema. 



Goniohasis, Schizostoma and Anculosa, are cer- 

 tainly distinct genera ; the first tvvo approximate, 

 forming the Goniobasic group or section ;* and the 

 last forms a section by itself, characterized by an 

 entire aperture. 



Yet this arrangement is liable to exception, as all 

 the species of a genus do not fulfil the ideas here 

 conveyed. Some species, on the contrary, remind 

 one of genera which do not immediately succeed or 

 precede them. Moreover, anatomical researches 

 will enable us probably to separate the natural 

 genera of this family much more sliarply than we 

 are now doing, and may enable us to seize on 

 corroborative characters of the shell, which are now 

 overlooked, or whose importance, in this connection, 

 has been thus far under-estimated. 



* Euryccelon "will be retained as a genus in this work although I 

 suspect now that the species should merge into Goniohasis and 

 A)i<-nlosa. Meseschiza, as I am convinced, represents an abnormal 

 condition of growth in very young shells from a single locality. 

 Unlilie Schizostoma, there is in Meseschiza every evidence that injury 

 to the shell causes the slit in the body whoi'l. In this case also I 

 retain the genus, simply because otherwise I should not know where 

 to place its single species. — May, 1872. 



