MORPHOLOGY OF THE HBLICINIDiE. 799 



course of disappearance. The median tooth is heart-shaped, 

 minute, and similar])^ feebly corneous. Lucidella (PI. XL. fig. 59) 

 is quite distinct in radular characters : in the lateral teeth the 

 stalk is practically obsolete, the bulk of the tooth consisting 

 mostl}' of the aliform pi'ocess with its thickened denticulate 

 border, which is continued posteriorly into the ai'ticular knob. 

 There is no anterioi' articular excavation, but a thin tiiangular 

 external piece which serves to support the articular knob of the 

 tooth of the row next in front, and for the attachment of the 

 external process. The third admedians are of the usual petaloid 

 shape ; the second admedians stout, triangular, with a thickened 

 curved anterior edge, bearing on its outer surface a small minutely 

 denticulate trenchant process. The first admedians are rather 

 large, but feebly corneous, with a thickened anterior non -denti- 

 culate border ; they have been modified in a manner analogous 

 to what has been obsei'ved in Orohophana. The medians are 

 broadly heart-shaped, feebly corneous, nearly divided into two by 

 a deep median antei'ior notch. 



Summing up these details and taking into comparison Troschel's 

 figures, which are mostly of species of Helicina sensu restiicto of 

 Wauner, and confining oui- attention to the lateral tooth, which is 

 the lai'gest and obviously of most functional importance in the 

 Neritacea as well as in the Helinacea, we see that there is an easy 

 transition from Etitrochatella to Alcadia : that the lateral tooth 

 of Alcadia is of the form characteristic of the Helicinidfe in 

 general, but shows a tendency to a reduction of the stalk, which, 

 as Troschel has shown, is common to many American and West- 

 Indian species. This tendency is exhibited in an extreme form 

 by Lucidella. But in the Pacific and Oriental genei-a the stalk 

 and the articular excavation connected with it are well developed. 

 But the lateral tooth of Eutrochatella bears an extremely close 

 resemblance to that of the Neritida?, and there is this further 

 resemblance, that the first admedian tooth, which is of veiy large 

 size in the Neritidfe, is relatively of much larger size as compared 

 with the second and third admedians in Eutrochatella than in 

 any other Helicinid. If such characters can be relied upon as a 

 guide to affinity, Eutrochatella is the most closely related among 

 the Helicinidfe to the Neritoid ancestor of the group. From 

 Eutrochatella forms have been derived : on the one hand, the 

 Proserpinidfe, which also have a lar-ge pileifoi-m lateral tooth ; 

 on the other hand, Helicina. We may infer that the earliest 

 Relicince retained the stalk and articular excavation which are 

 such mai'ked features in the pileiform lateral tooth of Euti'o- 

 chatella and Proserpina. 



The forms which, as suggested in the earlier part of this paper, 

 were transported by !■ some unknown means across the Pacific 

 Ocean to the Philippines must have possessed these features 

 and transmitted them unchanged to their descendants which 

 now inhabit the Oriental and Indo-Pacific regions. But in 

 America and the West Indies there has been a tendency, more 



