GUDE : ON THE ARMATURE OF LAND MOLLUSCA. 53 



During my investigation of these armatures in Corilla and 

 Plectoinjlis I discovered that in most cases the barriers in immature 

 shells differed considerably from those found in full-grown ones, 

 mbre especially in those of Corilla, while in one case, i.e. Corilla 

 adarnsi, these protective structures occur only in the immature 

 shells, the animal dispensing with them entirely on completing the 

 shell. Without knowing the actual conditions in its surroundings 

 it is, of course, impossible to account for this phenomenon, but it 

 may be surmised that the absence of predatory insects may have 

 produced this result, and that the formation of the barriers in 

 immature shells is simply the survival of an ancestral character. 

 Two other forms of protective structures, even more efficacious, 

 are found (a) in the members of the genus Clausilia, which produce 

 the elastic shutter, or clausilium, and (6) the numerous operculate 

 genera, whose members are provided with a lid, or operculum, 

 completely closing the shell. 



The first group to be considered in detail is the family of 



TESTACELLID^, subfamily STREPTAXIN^. 



Genus Streptaxis, Gray. 



This genus ranges through South and South-Eastern Asia, the 

 Mascarene Islands, tropical Africa, and South America. Several 

 species are devoid of armature, such as the helicoid forms : S. 

 wagneri, Pfr., and S. apertus, Mts., from Brazil, and elongate ones 

 such as : S. contusus, Fer., from Brazil, S. dacostce, Gude, from 

 Colombia, and S. nohilis, Gray, from Liberia. In the simpler forms, 

 such as 8. hurmanicus, Blanf., from India, S. pfeifferi, Zel., and 

 S. andamanensis, Bens., from the Nicobar and Andaman Islands, 

 only a raised, entering, parietal lamella is found, but the majority of 

 species have comj^licated obstructions at the aperture. For instance, 

 S. theobaldi, Blanf., from the Khasi Hills, has two raised lamellae 

 on the parietal callus, three on the basal lip and three on the outer 

 lij) of the peristome, while S. paulus, Gude, a Chinese form, has two 

 of the parietal callus, two on the basal and two on the outer lip of 

 the peristome. A curious helicoid form, S. roehelini, MUdff ., from the 

 Samui Archipelago, has a raised lamella on the parietal wall and three 

 palatal teeth ; this species belongs to the section Odontartemon. 



The genus Systrojjhia, Alb., confined to South America, is not 

 provided with teeth, but the parietal callus is raised into a curved 

 plate, in some species, such as S. systrophia, Alb., from Bolivia, 

 closely approaching the upper and loAver lip of the peristome, leaving 

 only a narrow slit for the animal to protrude ; there is besides 

 a constriction behind the peristome. In S. cheilostropha, Orb., a 

 Brazilian species, there are in addition two denticles, one on the 

 upper and one on the lower lip of the peristome. 



In S. reijrei, Souv., from Ecuador, the parietal callus is only slightly 

 raised, but within, nearly one-quarter of a whorl behind the peristome, 



