1893.] NATUBAL SCIENCES OF PHILADELPHIA. 397 



and its radula was 1 "63 mm. long, 0-70 wide, with 88 transverse 

 rows. In a mature specimen of 15 mm. diameter, the radula was 

 3*85 mm. long, 1 '34 wide, with 112 transverse rows. The former 

 had (in the middle part) 20 - c - 20 teeth, of which five were laterals and 

 two very nearly so; in the latter part there were 30'C'30 in the 

 anterior, 31'C - 31 in the posterior part, about 10 of them laterals. 

 They are represented in figs. V and VI, and the form of the teeth 

 is essentially the same in both. The centrals have distinct side 

 cusps ; in the laterals there is not a trace of the endodont, while the 

 mesial cusp, adjacent to the mesodont of the "marginals," seems to 

 be an endodont, since it stands nearly isolated on the base in the 

 younger teeth, as shown in 29, fig. VI, and then (as in many other 

 species of this genus, of Patirfa, and of Helicidw generally), gradu- 

 ally coalesces with the mesodont, ascends upon it, and finally becomes 

 obsolete. There is, however, considerable variation as to this in the 

 distal laterals, even in the same longitudinal row, as shown in fig. 

 VI, 25-29, 



Polt/gyra (Stenotrema) hirsuta Say gave essentially the same result. 



As already said, the observations presented above have been made 

 on but few species, and not complete even in most of these. They 

 must be carried further. Yet the results obtained appear to be for 

 the most part new and valuable ; they may briefly be comprised in 

 the following conclusions: 



1. The radula in land mollusca is quite different in the various 

 stages of life, both as to the number and the shape of the teeth. 



2. As to number : there are very few teeth in the first formed 

 transverse rows, and new ones, forming new longitudinal rows, are 

 added at the lateral margins, at first rapidly, then at greater and 

 greater intervals. The development of the radula goes on faster than 

 the growth of the animal, comparatively, so that in young examples 

 it is larger in proportion to the size of the animal than in the adult. 



3. As to shape : the first formed teeth are of very simple shape 

 and by gradual transformation give place to teeth of the final form. 

 This transformation may go through different stages, or by different 

 modes in cases where the form becomes ultimately the same. 



4. The question whether the addition of new teeth and the trans- 

 formation of teeth goes on as long as life lasts, or ceases with ma- 

 turity is not yet decided. 



