MORPHOLOGY OF TEETH. Xlll 



15, figs. 3, 4), although in some forms more cusps remain on the 

 outermost teeth. The individuality of these three cusps is remark- 

 ably fixed ; for however completely the typical tricuspid form may 

 be changed, it is always possible to identify the three primitive ele- 

 ments, or such of them as are retained. 



In the study of Helicid radulre, and especially those departing 

 widely from the typical structure, it is essential to recognize at the 

 outset 



The law of mesometamorphosis : All modifications in the' teeth 

 proceed from the median line of the radula outwards toward the edges, 

 the outer marginal teeth being the last to be modified. 



A study of the marginal teeth, therefore, gives a clue in many 

 cases to the ancestral condition of a much modified radula ; although 

 in certain groups the change has been so long established and has 

 proceeded so far that even the outermost teeth no longer retain 

 their primitive form. In such cases recourse must be had to the 

 radulte of young individuals or embryos still unhatched, which 

 sometimes retain an ancestral type of teeth (see Sterki, Proc. Acad. 

 Nat. Sci., Phila,, 1893, p. 388). 



The evident reason why the order of tooth-changes stated above 

 should obtain, is that the median portion of the radula is the part 

 most used on account of its position and the convex boss-like shape 

 of the subradular cushion. 



The most frequent departure from the tricuspid type of tooth is 

 seen in the lateral teeth of most Helices, in which the inner cusp 

 (eutocone) is lost, or more commonly its cusp is united with that of 

 the middle cusp (mesocone) as a lateral extension of the latter. In 

 many groups both inner and outer cusps of rhachidian and lateral 

 teeth are suppressed in this manner (see pi. 34, fig. 9), but all three 

 cusps reappear on the marginal teeth, which are less modified. 

 Usually the outer marginals have the ectocoue, or outer cusp, split 

 or bifid, a reminiscence of the early multicuspid teeth which were 

 part of the heritage of the Pulmonates from their Tectibrauch 

 ancestors. 



Radulce with teeth tricuspid in whole or part. In many Endodonti- 

 dce and minute forms of other groups, the teeth are all tricuspid (see 

 plates 8, 9). This form of teeth is usually correllated with small 

 size and strictly terrestrial habits. 



Radulce ivith all teeth unicuspid. In a few genera the loss of side 

 cusps has extended to even the outermost teeth of the radula (see- 



