574 THE VOYAGE OF H.M.S. CHALLENGER. 



the sixth pair of pleopoda, whereas in Synalpheus there are two, arid the telsori is 

 quadrate in the former and ovate in the latter, being fringed with strong spines or 

 teeth in Paralpheus as well as by numerous long hairs, whereas there are only 

 marginal hairs in Synalpheus. 



A strict comparison of the several features that distinguish Paralpheus from 

 Synalpheus induces me to believe that the distinction in the form of the mandibles 

 is the only one of true generic value, all the others only being points of greater or 

 less specific difference, which gradually lead us to the family of the Hippolytidae, 

 as illustrated in the genus Spirontocaris. 



Geographical Distribution. — This genus is limited to the only species known, and 

 that has only been taken in the narrow and shallow strait between New Guinea and 

 Australia. 



Synalpheus falcatus, n. sp. (PI. CIII.). 



Carapace broad, deep, longitudinally arcuate, and about two-fifths the length of the 

 animal ; dorsally elevated into a carina that commences over the gastric region and is 

 produced anteriorly to a rostrum that reaches as far as the distal extremity of the 

 second joint of the peduncle of the first pair of antennae. The orbital lobes are pro- 

 duced as sharp teeth that reach to half the length of the rostrum ; a deep groove 

 separates them from the median carina. 



The first pair of antennae has the three joints of the peduncle subequal, the first 

 being rather the longest, and carries a stylocerite that reaches anteriorly as far forwards 

 as the extremity of the rostrum ; it terminates in two unequal flagella, the outer being 

 about as long as the peduncle, and bifid at the extremity, and the inner being half the 

 length of the animal. 



The second pair of antennae has the basal tooth as long as the tooth of the orbital 

 lobe ; the scaphoeerite is pointed, longer than the rostrum, and shorter than the 

 peduncle ; the antennae terminate in a flagellum that is as long as the animal. 



The second pair of gnathopoda reaches about as far forwards as the extremity of the 

 rostrum. 



The first pair of pereiopoda has the larger chela on the left side ; in our specimens 

 of the female it is wanting, but in the male it is more than half the length of the 

 animal and narrow, being about three times longer than broad. The meros is armed 

 with a tooth at the upper distal angle ; the carpos is short and armed with a tooth at 

 the lower and upper angles ; the dactylos is broad, curved, and furnished with a large 

 tuberculose tooth near the joint on the under side, which, when closed, shuts into a 

 hollow. The smaller chela is very short, the meros is armed with a strong tooth at the 

 upper distal angle ; the carpos having a similar one at the upper and lower angles ; 



