• New Genus of Land Shells. 485 



form of a series of shells, as well as on a peculiarity in the animal alone; 

 especially when we consider how very few of the animals of the different 

 species which we are called upon to arrange are, or ever can be, known, 

 and also as we constantly find by experience that every peculiarity in 

 the form or structure of the shell is the indication of some peculiarity in 

 the habit or organic structure of the animal which formed it, and warrants 

 its separation from the rest of the species of the family. 



The antique lamp, on which Lamarck established his genus Anostoma 

 has been long known and valued, on account of its rarity and strange 

 form ; the animal turning up the last whorl before it completes its 

 growth, so that the mouth of the shell is even with the outer surface of 

 the spire. A similar form has been lately observed among the fossil 

 shells, which, on account of its resemblance to the Cyclostomatse, by the 

 roundness and simplicity of its mouth, M. Deshayes has separated into 

 a genus, under the name of Strophostoma. In my paper on the structure 

 of shells {Phil. Trans., 1833), I pointed out that some land shells, as Helix 

 contusa, when they arrive at a certain period of their growth, throw their 

 whorls out of the regular series, as if the shell had been crushed, pro- 

 ducing what may be considered as a natural distortion. Having, since 

 that time had the opportunity of observing several other species of a 

 similar structure, and finding that they all agree in the general form and 

 position of their mouth, I am induced to consider them as forming a 

 peculiar group, for which I propose the name of Streptaxis. They also 

 agree in the shell being generally concentrically striated, except in the 

 flattened part of the front of the hinder part of the last whorl, near the 

 mouth, which is smooth. One of the species forms, during the dry season, 

 a hard, thin, calcareous epiphragma, differing considerably in structure from 

 any that have hitherto been observed among the Hell cidae ; but they may 

 be only a peculiarity of the species, though the epiphragma in this family 

 often forms a good subsidiary character. 



60 



Genus Strepta'xis n. g. Animal like Helix.? Shell ovate or oblong; 

 when young, sub-hemispherical, deeply umbilicated, with rapidly enlarging 

 whorls. At length the penultimate whorl is bent toward the right and 

 dorsal side of the axis, and the umbilicules become compressed, and 

 often nearly closed. The mouth lunate ; the edge slightly thickened and 

 reflexed, and often with a single tooth on the outer side of the inner or 

 hinder lip. 



These shells inhabit the tropical rts of Africa and South America ; 

 and two of the species of these two distant countries appear to be very 

 nearly allied. 



N N 3 



