64 PROCEEDINGS OF THE ACADEMY OF [Vol. LXXIV 



Genus CALYBIUM Morelet. Indo-China. 

 Subgenus CALYBIUM s. s. 

 Calybium Morelet (1891). Type (monotype) C. massiei Morelet (1891). 



Subgenus GEOTROCHATELLA P. Fischer. 

 Geotrochatella P. Fischer (1891). Type (now chosen) H. mouhoti Pfr. (1862). 



In this group the radula of the type species C. mouhoti (Pfr.) 57 

 has been examined and is figured (Plate VI, fig. 29). It is startlingly 

 like that of Viana regina, but has several, rather important dif- 

 ferences. The A-central is not especially enlarged, as compared 

 to the two outer, paired centrals. The unicuspid marginals are 

 much broader, and the number of marginals is the smallest observed 

 (28), which is remarkable when one considers that the tiny E. 

 rupestris has 35. As the broad, outer marginals were observed in 

 both radulae of C. mouhoti, it does not seem possible that the 

 count is very erroneous. 



With the depressed shell, Geotrochatella has a linear operculum, 

 somewhat similar in shape to that of Priotrochatella. However, 

 the horny plate of the operculum in the former group is well- 

 developed, while the calcareous plate is extremely thin. The 

 spiral nucelus is near the columellar margin. The horny plate has 

 a thickening, which runs transversely across the inner side; when 

 examined under the microscope, this thickening is seen to consist 

 of lamellae, which project out a considerable angle to the rest of 

 the plate. As the margin is not thickened, as is the calcareous 

 plate of Priotrochatella, it appears as if the growth-stress, due to 

 the shape of the aperture, finds expression in this lamellar thicken- 

 ing across the middle of the plate. Thus the opercula of Priotro- 

 chatella and Geotrochatella, although superficially similar, are fund 

 mentally different, both in texture and arrangement of the growth 

 lamellae. For this reason and on account of the divergence of the 

 radulae, it seems that the relationship between these two groups 

 is not as close as has often been stated. They probably represent 

 parallel development in two rather distantly related stocks. The 

 dominance of the horny operculum also helps to separate this 

 group from Viana, which, it appears to me, is more probably its 

 closest relative in the West Indies. 



A. J. Wagner included Geotrochatella, Calybium and Priotrochatella 

 in his Pseudotraochatellinae. This "subfamily," altho based on 

 opercular characters, had for its type genus Pseudotrochatella 



67 2 dried specimens; A. N. S. P. no. 66060; collected at Luang-Prabang, Laos, 

 by Ph. Dautzenberg. (1895)- 



