TETHYS DACTYLOMELA (RANG) 19 



a dark amber color. As is usual the anterior rows of teeth are 

 very much worn and broken, so that even their number is un- 

 certain. About eleven such imperfect rows may be made out, 

 which gradually pass over into forty-seven complete rows, of 

 which the last twenty are still inclosed in the radula sheath. The 

 total number of rows is thus about fifty-eight. The greatest 

 length of the radula is 9.0 mm., and its greatest width 3.0 mm. 



The number of teeth varies in the different rows, being 

 quite small in the most anterior rows, and increasing regularly 

 in the succeeding ones to 26:1:26 in the twenty-fifth row, and 

 38:1 : 38 in the fifty-fifth one. In the anterior twenty to twenty- 

 five rows the teeth are much worn, scarcely any being perfect, 

 the cusps being usually blunted, or broken entirely away. Typical 

 teeth, taken at intervals across the radula from neighboring rows, 

 are shown in figs. I, 2 and 3, of PL I. Fig. I represents the 

 median and first lateral teeth of the 49th and 5oth rows; fig. 2 

 shows the 8th, 9th and loth teeth of the same rows, while figs. 

 3 and 4 give the i8th to 2Oth of the 5oth row and the 32d to 37th 

 teeth of the 54th row respectively all under the same magnifi- 

 cation. The rhachis bears a single large tooth in each row. Its 

 base is broad and trapezoidal in form, the broader end being 

 directed posteriorly. It varies but slightly in size throughout 

 the length of the radula, averaging 0.216 mm. in the diameter 

 of its broader posterior end and 0.06 mm. in the diameter of 

 its anterior end, with a length of 0.156 mm. In the anterior 

 end of the radula the base is often divided longitudinally, or 

 a thinning away of the median line may indicate such a division 

 as incomplete. The posterior margin is very slightly concave, the 

 anterior one deeply emarginate, the notch being carried up on the 

 back of the hook as a deep groove. The anterior end bears a 

 strong hook, which is as broad as the full width of the base at 

 its anterior end. The length of this hook averages 0.126 mm., 

 seven-twelfths the length of the base. It terminates in a large 

 median blunt cusp, and two much smaller lateral cusps. The 

 sides of the median cusp bear from four to ten thin irregular 

 denticles on either side in the posterior portion of the radula. 

 These denticles are either separate or, more usually, united at 

 their bases. In the anterior portion these denticles are either 

 worn away or undeveloped. The lateral cusps (PL I, fig. i), are 



