ZOOLOGICAL OBSERVATIONS IN THE SOUTH PACIFIC. 221 
dimorphism, which, if found to be constant among a large 
number of individuals, would be a fact of some importance. 
My two specimens alone prove that an appreciable sexual 
dimorphism may occur, whether as a variation or as a fixed 
property. 
The following are ali the numerical facts that I have ascer- 
tained regarding the two specimens. 
1. Female. Length, 29°5 mm.; number of myotomes, 60; 
formula, 40—9—11; length of tail or urostyloid process be- 
hind myotomes, 3 mm.; distance of anus from root of tail, 
2°5 mm.; distance of atriopore from root of tail, 6°5 mm. 
2. Male. Length, 20 mm.; number of myotomes, 64; 
formula, 44—9—11; number of gonads, 30, disposed uni- 
laterally on right side. 
The characteristic generic features of Asymmetron, exhibited 
by the new species, are as follows.! 
The right metapleur is continuous with the ventral fin; the 
latter has no fin-chambers and no fin-rays; the right and left 
metapleura pass equally into the rostral fin in front ; an inter- 
tentacular membrane is present betweenthe ventral buccal cirri ; 
the buccal cirri are plain; and lastly, there is a pair of post- 
atrioporal, or, better, post-atrial caeca, which are, however, very 
short, ending abruptly between the posterior lips of the 
atriopore. 
The dorsal fin-spaces become very irregular posteriorly, and 
cease at about the 54th myotome. 
In addition to the length of the body and the number of 
myotomes, an important specific feature of Asymmetron 
caudatum is the rostral fin, which is marked off from the 
dorsal fin by a pronounced constriction of somewhat varying 
depth (cf. figs. 2 and 4). The rostral fin is indeed a striking 
feature in the fresh object, but is less marked after preservation. 
The caudal fin also varies considerably in the two individuals, 
its urostyloid portion being much more constricted off from 
the succeeding portion in the region of the myotomes in the 
male specimen than in the female (figs. 1 and 3). It is, how- 
' T have verified all the points mentioned by sections and preparations. 
