1893.] NATURAL SCIENCES OF PHILADELPHIA. 391 



THE CENTRAL TEETH. 



The central tooth is first formed as a simple, roundish or oval 

 nodule, with a blunt point directed backward (fig. I and VI), 

 rapidly increasing in length in the following rows. At the same 

 time one or two somewhat irregular side cusps appear on either side, 

 which on about the eighth tooth become more constant, more marked, 

 and single on each side. There are, however, occasional exceptions 

 as shown in the tenth of fig. VI, where on the right 4 side there are 

 again two side cusps. Thus it attains its final shape, but is some- 

 what unsymmetrical, and so it remains, not only in one specimen, 

 but as a rule in all (see figs. II- VI). This is also the case in other 

 species. In some individuals the right side cusp is more posterior, 

 in others the left. It early becomes noticeable that the central tooth 

 stands more posteriorly than the laterals on either side of it, as repre- 

 sented in fig. VI. Comparatively early the plate of attachment 

 extends backward, at first quite small, barely noticeable, and only 

 on the side bearing the anterior side- cusp; but later it extends on 

 both sides. 



These facts make it evident that the central tooth is simple prim- 

 arily, not formed by the coalescence of two original laterals as has 

 been supposed. 



THE LATERAL TEETH. 



An adult specimen of Limax campestris has about 11 to 18 5 lateral 



teeth. And to my surprise I found that, although they are exactly 

 alike in their final shape, they are formed in two widely different 

 ways : 



(1). The 6 or 7 mesial laterals on each side first appear as thin, 

 simple, transverse, nearly rectangular plates, each as a rule preceded 

 by a short, nodule- like "trace," (see figs. I and VI). They soon 

 become longer at their inner ends, more triangular, and the mesodont 

 forms, being at first short and blunt, but immediately growing longer, 



4 Right and left are here given as they appear under the microscope. 



5 These limits are exceptionally wide and may be considerably narrowed by 

 subsequent careful observations. The specimens with fewer laterals may very 

 likely be immature. In the slugs there is no definite criterion of maturity, except 

 probably in the genital organs, which should be consulted in eveiy case. W. G-. 

 Binney found the tooth formula to be c. 18'22 in one specimen, c. 1125 in another ; 

 I counted c. 14xT31 in au individual 27 mm. long but still probably not fully 

 mature. 



