1893.] NATURAL SCIENCES OF PHILADELPHIA. 389 



attention will show that in a certain longitudinal row at the margin 

 of the " central field," a transition tooth gradually transforms into a 

 well shaped lateral, a marginal into a transition tooth, and finally, in 

 turn, into a lateral. 



In these few lines the development and transformation of the radula 

 is briefly outlined. This organ is not, as has generally heen supposed, 

 always of the same formation, number of teeth, etc., in a given 

 species, and only smaller in the young examples, but there are 

 actually fewer transverse rows in the young, and fewer teeth in each 

 row. Moreover, the teeth at first are of very simple form, and by 

 gradual changes by replacement they assume their final shape. The 

 transformation is never simple, but is accomplished through one or 

 several intermediate stages, sometimes widely different ; and more- 

 over, the process differs widely in the various species and groups. 



The general considerations above presented are based upon the 

 examination of the few species with which the present communica- 

 tion deals. This paper must be regarded as the beginning of a long 

 series of examinations, which should comprise all species. Such a 

 research will doubtless give results not only highly interesting in 

 themselves, but also of great taxonomic value. Certain characters of 

 the radula which are very indistinct in the adult, or even overlooked 

 by careful observers, become of characteristic value in the light 

 gained by the examination of the earliest stages of growth. In the 

 embryonal animal we find forms of teeth present then and there 

 only, disappearing entirely in post- embryonal life ; so we may 

 properly speak of a true metamorphosis of the radula. 



The species examined with reference to this metamorphosis are as 

 follows: Limax campestris Say ; Zonitoides arborens Say ; Z. ligerus 

 Say; Zonitoides sp. undet. from Tennessee; Z. suppressus Say; 

 Patula striatella Anth.; altemata Say ; solitaria Say; Polygyra (Trio- 

 doptis) tridentata Say ; P. (Sienotrema) hirsuta Say. Of most of these 

 I had no embryos at hand for examination, but very young specimens. 



Limax campestris Say. PL 10. 



This species, and probably all of the genus, are very favorable sub- 

 jects for our purpose, since oviposition is continued throughout the 

 season, so that ova, young, adolescent and adult examples may be 

 found and compared at almost any time. The ova are not usually 

 deposited in clusters consisting of great numbers, as in most other 



