MUSEUM OF COMPARATIVE ZOOLOGY. 349 



cept that the rbachidian tooth, instead of having a comparatively small base 

 and a distinct and well developed cusp, has a large base, which is thin and of 

 rounded shape, with an almost obsolete cusp. Many of these teeth could 

 hardly be said to be cusped at all, and in the best developed unworn ones 

 the cusp is but little raised, small, composed of a median rounded portion and 

 on each side of that a single obscure denticle (see figure). The whole tooth, 

 in most cases, resembled a mere thin scale, and might readily be overlooked, 

 or regarded as not a true tooth at all, in some specimens. 



The most extraordinary feature is peculiar to the males, of which four speci- 

 mens were examined and agreed perfectly with one another. The right tenta- 

 cle is somewhat stouter than the left one, but near the base on the outer side 

 both have a distinct bulb or elbow-like prominence similar to that which in 

 many gastropods bears the eye. A careful examination of sections, however, 

 did not disclose any nerve, rods, or optic cup, and it is certain, if the last 

 exists, that it is unprovided with any pigment cells. The cutis seemed of a 

 uniform and rather unusual thickness, partly due, probably, to the contraction 

 caused by the alcohol. The tip of the contracted right tentacle was rather 

 broad and blunt; it also seemed somewhat flattened. From the outer side pro- 

 jected a verge of tentacular form, more slender, slightly longer, and more 

 flattened than the tip of the true tentacle, and horizontally recurved. From 

 the circular wrinkles caused in it by contraction it seemed capable of much 

 elongation. In the anterior edge near the tip, but not extended over the tip, 

 was a deep fissure extending to the centre of the organ and along its anterior 

 edge from the tip backward a distance a little more than equal to the width 

 of that part of the organ. The proportional length of this fissure differed in 

 different individuals a little. From the depth of this fissure a tube or canal 

 extends through the centre of the penis, its diameter being about one sixth 

 that of the organ. At a point in this canal not far from the junction of the 

 verge with the tentacle there appears to be a subtriangular fossa, from which 

 the tube continues toward the body at about the same distance from the inner 

 surface of the verge, and of about the same size as before the fossa was reached. 

 The canal passes on into the base of the tentacle, where it is lost to sight in 

 the solid opaque tissues. The extremely small size of the animal combined 

 with its coarse, and (by alcohol rendered) very opaque tissues, prevented the 

 more thorough examination which would have been practicable with fresh 

 specimens or those of larger size. Nevertheless, it does not appear that this 

 organ can be anything else than an intromittent male organ, such as is now 

 known to exist in Addisonia, in Neritina, in some species of Cranopsis, and 

 other deep-water Rhiphidoglossa. 



At the time I made this discovery the only form in which an external in- 

 tromittent organ had been reported among the Rhiphidoglossa was Neritina, 

 and even this fact, though easily verified, had been questioned. 



The verge in the Neritidce, however, is broader and rounder, flatter, and 

 shorter than in Cocculina, and shows its nature less clearly in its external 



