25 



/d I 



The upper part of the dorsal margin of the post-abdomen is 

 prolonged, and runs nearly parallel to the posterior margin of the 

 valves, while the lower part is parallel to the ventral margin, and 

 makes nearly a right angle with the upper part. The apex is 

 rounded and bears two small terminal claws, each of which has a 

 small basal spine. 



The general shape of the post-abdomen is like that of a shoe, 

 whence the generic name. It is much compressed laterally, and 

 its armature consists of numerous bristles scattered somewhat ir- 

 regularly over its surface. 



The usual pair of setae project backward from the post-abdo- 

 men ; and the ventral margin of the valves is fringed with some- 

 what long, plumose setae. 



The antennules are of moderate size, do not reach to the end of 

 the rostrum, and have a flagellum and eight to nine sense-hairs. 

 The antennas are about as large as in Pleuroxus, and bear eight 

 setae (|tt) and three spines (-J--8-J-). The keel of the labrum is some- 

 what prolonged backward, as in Pleuroxus, but not to so great an 

 extent. 



The eye, macula nigra, intestine, anal coecum and shell-gland 

 present no points of especial interest. 



The shape from above is an elongated oval, broadest through 

 the fornices. The valves gape considerably below. 



The animal moves by strong and sudden blows of the anten- 

 nae. With a single stroke it darts for a short distance, and then 

 returns to rest, ordinarily not moving again until disturbed. It 

 may, however, swim for a considerable distance by repeated 

 strokes of the antenna?, but on the whole is decidedly sluggish, 

 remaining for hours at rest. The extreme suddenness with which 

 it starts into motion is very remarkable. The antennas are ordi- 

 narily bent downward along the anterior margin of the valves, 

 and I have never been able to see them raised preparatory to a 

 saart. It passes instantaneously from rest to motion, and, with- 

 out any warning, is gone from the field of view in the microscope. 

 It can also move slowly, as I have noticed, by strokes of the ab- 

 dominal feet upon the cover of the live box in which it is kept. 

 The post-abdomen does not seem to be employed as an aid to lo- 

 comotion. 



