NO. 5.] ACCOUNT OF THE SPECIES. 



131 



the rod-like portion of the latter. This stem seems to be composed of 4 

 joints, which, however, are far from being distinctly defined. The first 2 

 joints are rather elongated, and of about equal size, the 2nd carrying above, 

 at some distance from the tip, a slender anteriorly-curving seta. The 2 distal 

 joints are very small, and curve abruptly downwards, forming together a 

 short terminal part. To the end of this part, 4 subequal and very delicate 

 sensory filaments are attached, and in fi-ont of them a very slender seta, 

 which projects far from the tip of the rostral prominence. Within the basal 

 joint, a number of irregularly arranged lenticular bodies of a dark brownish 

 colour may be traced, imbedded in a ganglionic mass. These bodies, which 

 also occur in other species of this genus, seem to I'epresent a sort of imper- 

 fect visual organs. 



The antennae (fig. 5) are very powerfully developed, constituting the chief 

 iocomotory organs of the animal. They each consist of an exceedingly large 

 and broad basal part, and 2 very unequal rami. The basal part almost 

 attains half the length of the shell, and is extended anteriorly. It is oblong 

 triangular, or obpyriform in outline, with the hind extremity very broad and 

 somewhat obliquely rounded, the anterior tapering gradually. Its inner face 

 is applanated, whereas the outer is convex, and within it numerous strong 

 muscles are seen converging to the anterior extremity, and chiefly acting 

 upon the outer, or natatory ramus. The latter is very movably articulated to 

 the end of the basal part, and somewhat exceeds half its length. It is nar- 

 row cylindric in form, and divided into 7 joints, the first of which is about 

 3 times as long as all the others combined. The latter form together a well- 

 defined and very flexible terminal part of oblong fusiform shape, carrying, in 

 all, 9 densely plumose natatory setae, which successively increase in length 

 proximally, and admit of being spread out in a fan-like manner. On a closer 

 examination, these setae are found to terminate in a naked lanceolate point of 

 membranous consistency, and probably sensory in character. The inner, or 

 accessory ramus is attached at some distance from the tip of the basal part 

 inside, and, as a rule, extends downwards. It is composed of only 2 joints, 

 the 1st of which has the form of a rounded, membranous lamella projecting 

 anteriorly into 2 small successive prominences, the proximal one quite simple, 

 acute, the distal one more prominent, and carrying 2 short setae. The last 



