42 OPISTHOBRANCHIATA OF BRAZIL 



row increases from 16:1 :i6 in the oldest complete row to 22 :i :22 

 in the thirtieth. The dental formula of this individual may be 

 expressed then as 38x16-22 :i :i6-22. The rhachis bears a single, 

 massive tooth (PI. IV, fig. 23; PI. V, fig. 26, w), its base, trap- 

 ezoidal in form, measuring 0.27 mm. in width at its posterior 

 end and 0.132 mm. in length, varying but slightly from these 

 dimensions throughout the length of the radula. The posterior 

 margin is slightly emarginate, the anterior one deeply so, the 

 curve being carried up on the anterior face of the hook as a 

 broad, deep groove. The anterior end is prolonged upward and 

 backward in a strong hook, terminating in a stout, triangular, 

 median cusp, upon either side of which are borne two smaller 

 cusps typically. The larger of these, next to the median one, is 

 from one-half to two-thirds the length of the latter; the outer 

 ones are very much smaller and are more variable in both form 

 and size. 



With the exception of the outermost three to five teeth, the 

 lateral teeth are strongly hooked and of similar form, decreasing 

 gradually in size toward the outer borders of the radula (PI. IV, 

 figs. 23-25). Each lateral tooth (PI. IV, figs. 23-25; PI. V, figs. 

 26-30), consists of a stout oblong base obliquely placed, from 

 the dilated anterior end of which arises a stout hook, terminating 

 in a triangular cusp. The sides of this cusp bear four to ten 

 denticles, quite small near the tip and, in general, increasing in 

 size toward the base, though irregularities in this are not infre- 

 quent. Upon the inner side of each tooth, at the base of the hook, 

 the series of denticles is terminated usually by a very large and 

 broad denticle, while upon the outer flank of the main cusp a 

 series of two or three smaller cusps is borne. In the radula of 

 Tethys dactylomela there is but one external cusp in this position, 

 which is usually itself denticulate, but in T. cervina the margins 

 are uniformly smooth (cf. PI. I, figs. 1-5, and PI. IV and V, figs. 

 23-30). Indications of a fourth lateral cusp at the extreme end 

 of this series are frequently found. The first ten laterals are 

 approximately equal in size (PI. IV, figs. 23-24, i-io), the re- 

 maining ones decrease toward the outer border of the radula (PI. 

 IV, fig. 25), the hook finally becomes rudimentary and disappears 

 altogether, the outermost three to five teeth being reduced to 

 oblong flattened plates (PI. V, fig. 28). The teeth of this species 



