1903.] NATURAL SCIENCES OF PHILADELPHIA. 205 



thin and fragile; internally having obliquely radial laminae, or rows or 

 pairs of teeth, at intervals of a third of a whorl, some or all of them 

 often wanting, especially in old individuals. 



Genital system without dart sac. Radula with teeth of the central 

 row tricuspid; two or three laterals on each side also tricuspid, the 

 entocone raised high upon the mesocone, of which it forms a lateral 

 spur. Marginal teeth of the usual simple and thorn-like form. 



Snails of this group have the discoidal shape and closely coiled whorls 

 of the typical (European) section of Vitrea, and they have also a den- 

 tition of the same type, which is remarkable for the peculiar mode of 

 specialization of the lateral teeth. Paravitrea differs from typical 

 Vitrea in the umbilicate axis and the development of teeth. The 

 compact coiling of the numerous whorls sufficiently distinguishes 

 Paravitrea from the Hyalina type of Vitrea, such as V. hammonis, etc. 



The species of Gastrodonta protected by teeth, have only a single pair 

 near the aperture, constantly added to in front and absorbed behind 

 with growth of the shell ; but in Paravitrea successive sets are formed to 

 be absorbed later. 



The adult or old individuals of species of Paravitrea usually absorb 

 all the teeth, and form no new ones in the latest stages; or in some 

 cases, as in V. capsella, teeth may be formed in occasional or rare 

 very young individuals, while in the intermediate and later stages of 

 growth none are developed. In a few other species, such as V. clappi, 

 V. sim.psoni and V. placentula, no teeth have yet been observed even 

 in the young. If my interpretation of the facts is correct, such species 

 as V. andrewsoe and multidentata, which commonly possess teeth in 

 adults, are old, relatively unchanged types; while forms toothless at 

 all stages are the most evolved. Paravitrea tJius consists of species 

 and races in various stages of reduction and loss of teeth, but descendants 

 from an ancestral stock which had them. 



Another modification of the ancestral radially toothed Paravitrea 

 is seen in certain species in which even-edged or serrate radial ribs or 

 lamellae replace the rows of teeth. The irregular or serrate edge of 

 this lamella in some individuals indicates that it has been formed by 

 coalescence of a row of teeth, the intervals between them becoming 

 filled up, exactly as in the Clausiliidoe of eastern Asia the lunella has 

 been formed by coalescence of a primitive row of palatal plicifi.'* In 

 this phylum, too, the armature has been lost in some species and in 

 some individuals of species normally toothed; and by acceleration, ex- 



® See these Proceedings for 1901, LIII, p. 638. 



