X TEETH OP ACHATINELLID^E. 



more remotely related to Partulidce. The structure of the 

 pallial organs and kidney is very similar in the first three of 

 these families. The male reproductive organs also, while dif- 

 fering in details, are much alike, having features widely pre- 

 valent in the lowest land snails, also in Onchidium, etc. 



Partula, by lacking an appendix on the penis, and also by 

 the simple prostate gland and the somewhat different form 

 of kidney, stands a little apart. 



By the structure of the female reproductive system, the 

 Achatinellida are most nearly akin to the Amastrida, but are 

 more evolved by the reduction of the albumen gland to a 

 minute rudiment, the prostate being at the same time enorm- 

 ously enlarged. The difference between the two families in 

 this character is one of degree rather than of kind. 



From all other families mentioned above, the Achatinellidce 

 differ strikingly in dentition. Those families have teeth of 

 normal shape, with cusps resembling and homologous with 

 these of the Helicida, Lymnaida, and most other Pulmonate 

 snails. The teeth of Achatinellidce and fornatellinidce differ 

 strikingly by having basal-plates of a different shape; 

 and their cusps are not recognizably homologous with those 

 of other land snails. Moreover, there is no division into 

 lateral and marginal areas of the radula with different forms 

 of teeth. 



Morphology of Achatinellid teeth. 



It is well known that arboreal snails usually have the teeth 

 specially modified, more or less unlike those of their nearest 

 relatives among ground snails. Liguus and Polymita are not- 

 able examples, but in many others, such as Drymceus, Papuina, 

 etc., a process of change is evidently in progress. In all of 

 these genera, the cusps, though variously changed, are still 

 clearly homologous with those of normal teeth. Without com- 

 mitting ourselves to any special hypothesis of how such 

 changes come about, we cannot avoid the conclusion that in 

 some way the nature of the food or the conditions of grazing 

 lead to adaptive remodelling of the teeth in arboreal snails. 

 So long as the modification does not affect the homologies of 



