56 THE NAUTILUS. 



of the genus thus defined have been distributed by New Zealand 

 authors into the following groups which they regard as genera : 

 Gerontia Hutt., Therasia Hutt., Thalai<.v'a Alb., Pysra Hutt., 

 Pyrrha Hutt., Phenacohelix Suter, Putulojixix Suter, Ampliidoxa 

 Alb. and Calymiia Hutt. These sections or subgenera are founded 

 upon various modifications of the shell or jaw, but they have not 

 sufficient distinctness to rank as genera, unless we understand that 

 term in a much more restricted sense than it has been used by the 

 majority of conchologists or zoologists generally. These minor 

 divisions are however natural groups and they are useful if we do 

 not overestimate their importance. The sections or subgenera of 

 this genus may stand as follows, the sequence of names being 

 chronological.^ 



Gerontia Hutt., 1883 (Type G. pantherhia Hutton.) 

 . Therasia Hutt., 1884, (Type C. celiiide Gray.) 



Calymna Hutt., 1884, + Amphidoxa Hutt. not Alb. (Type C. 

 cosfulata Hutt. 



Pyrrha Hutt., 1884, (Type P. cressida Hutt.) 



Phenacohelix Suter, 1891 (Type H. pilula Rve.) 



Allodiscus Pils., 1892 (Type H. dimorpha Vh.).=Pysra Hutt., 

 1884, non Stal, 1876. 



Suteria Pils. 1892 (Type H. ide Gr&y).:=^ Pat ulops is Suter, 1891, 

 non Strebel, 1879. 



ThalassohelixFih. 1892 (Type H. zelandice Gray).=:Thalassia 

 Hutton (? and of Albers,) not Thalassia Chevrolat, 1834. (Coleopt.) 



It will be noticed that Amphidoxa has been dropped or rather 

 united to Calymna. The true Amphidoxa has not been found else- 

 where than upon the island Juan Fernandez and the neighboring 

 South American Coast. I have compared specimens with the New 

 Zealand shells and find that there is not the slightest ground for 

 supposing them congeneric. I am disposed to believe that the New 

 Zealand Thalassias do not belong to the same genus as the x^ustra- 

 lian suhrugata Pfr., the type of Thalassia Albers. The other 

 departures from the usage of New Zealand authors are sufficiently 

 explained in the above list. 



1 The genus Phacussn of Hutton is included by Suter. It may prove that the 

 Zonitoid aspect of the dentition of that form is a secondary modification and not 

 truly Zonitoid. In this case the i^roup will be included among the present forms; 

 otherwise it must remain in Zouitidae, where Hutton placed it. 



