158 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



bends backwardand so passes beneath the tongue to near the (jesophagus. (Fig. 3.3, RS). Grooves 

 run inward from each side of the tongue to the opening of the radular sac, giving it a V shape. 



The radular sac (Fig. 33) is about 2.5 millimeters in length and has an average width of 8 

 millimeters. Its ventral and lateral walls are thin and transparent and allow the l)rown color of 

 the radula to show through. The extreme posterior portion of the sac, however, is colorless, 

 since the teeth are formed here and have not yet taken on their color. This part of the sac also 

 is somewhat wider than the rest, for the radula, at first spread out flat, l)ecomes more and more 

 iiirolled at the edges as it grows forward. From each postei'ior corner of the radular sac a 

 ligament of elastic fibres runs back to the muscular membrane near the (esophagus, evidently 

 tending to hold the radular sac in place (RL). The dorsal wall of the radular sac is quite thick, 

 and tits closely upon the teeth. In the anterior portion, where the radula is inroUed, the dorsal 

 wall of the sac projects to occupy the groove. As the radula passes out of its sac a plain 

 chitinous liorder, 3 to 4 millimeters wide, is added upon each side. (Text tig. 8.) This l)order 

 appears to be formed at the bottoms of the grooves running back from the opening of the 

 radular sac. 



Let us now examine the separate teeth of the radula. The teeth are arranged in rows 

 running across the chitinous, ribbon-like base of the radula; thei'e are in all about lifty of these 



I'ows. In each row are thirteen teeth symmetrically arranged with 

 regard to the central tooth. Each of the six lateral teeth is exactly like 

 the corresponding tooth of the opposite side in shape and position. It 

 scarcely seems necessary to give a detailed description of each tooth 

 after the careful descriptions and figures of Vayssiere and with the figures 

 accompanying this paper. I should not have drawn new figures of the 

 teeth if Vayssiere had not used the shape of the lingual teeth as a charac- 

 ter to distinguish between the two species of Nautilus, JV\ jwmp/lkis 

 and jV. macrcnnphalus. But when I found as great difl:'erences in shape 

 between his figures of the teeth of .Y. jMiiijiiUm and my specimens of 

 the teeth of the same species as he shows between the teeth of the two 

 species it seemed to me to be wise to make new figures for the sake of 

 comparison. All the teeth are very firmly attai'hed to the chitinous 

 ribbon by large bases. The bases of the central seven teeth are approxi- 

 mately scjuare, while the bases of the other teeth are much longer than 

 wide. The projecting tip of the central tooth is directed straight back- 

 ward; the other teeth while projecting l)ackward are also directed more 

 or less inward toward the central tooth. Let us immber the teeth from 

 the central tooth outward, distinguishing them as first lateral, second 

 lateral, and so on. 



Iiumediately outside the central tooth (Fig. 42) is a tooth very much 

 like the central one but smaller and having a shorter and a much less slen- 

 The second lateral tooth has a broader base and a much shorter, ])lunter 

 point, which is directed toward the central tooth. (Fig. 44.) The third lateral tooth (Fig. 45) is 

 long and slender and curved. Attached by a base which makes an obtuse angle with the body of 

 the tooth, it projects backward and inward so as to cover the first and second lateral teeth of 

 more posterior rows. The outer (anterior) face of the tooth is gouged so as to form a sharp 

 edge running nearly parallel to the inner outline of the tooth. The fourth lateral tooth (Fig. 46) 

 is rod-like and attached to the ribbon along its entire length. Its dorsal surface is sharply 

 ridged for nearly the length of the tooth; for part of its length the ridge is produced into a thin, 

 sharp, transparent blade of chitin which also projects backward. The fifth lateral tooth (Fig. 47) 

 is very much like the third, but longer, more slender, slightly less curved, and often more 

 sharply pointed. It is similarly gouged out ujaon the outer surface, but the sharp edge is not so 

 close to the inner edge of the tooth as on the third lateral tooth. This tooth overlaps the fourth 

 lateral tooth and the base of the third lateral tooth in more posterior rows. The outermost 

 tooth of all is also rod-like, attached to the ribbon along its entire length, and bears a slight 



Text-fig. 8.— Outline of the nui- 

 uhir ribbon. The dotteti lines 

 iiidieate the position of the 

 outer rows of teeth. 



der point. (Fig. 43.) 



