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



Tribe Monocarpidea. 



The genera in this tribe consist of those forms that differ from the Polycarpidea in 

 having the carpos of the second pair of pereiopoda formed of a single joint, and generally 

 the chela of this pair larger than that of the first. As in the preceding tribe the genera 

 may conveniently be divided into those which have the first pair of pereiopoda mono- 

 dactyle and those in which they are didactyle, or chelate, and to this tribe may also be 

 added a third division, comprising those in which all the pereiopoda are monodactyle. 



Family Thalassocarid^e. 



Carapace dorsally smooth, anteriorly produced to a laterally compressed rostrum. 

 Pleon narrow, laterally compressed and tapering to a pointed telson truncated at the tip. 

 First antennas having a stylocerite and terminating in two flagella. Second antennae 

 furnished with a scaphocerite that is rigid on the outer margin and armed with a tooth. 

 First pair of pereiopoda simple ; second chelate. Pleopoda foliaceous and biramose. 

 Rhipidura well developed. 



TJialassocaris, Stimpson. 



Tlmlassocaris, Stimpson, Proc. Acad. Nat. Sci. Philad., p. 42, January 1860. 

 Regulus, Dana, U.S. Explor. Exped., Crust., vol. xiii. p. 597 {nom. pi-xocc). 



Animal slightly compressed ; carapace more than one-third its length. Rostrum 

 elongate and dentate. Frontal margin produced to a tooth corresponding with the first 

 pair of antennae, but without a second antennal or frontodateral tooth. 



Pleon more compressed than the carapace ; somites subequal and laterally produced 

 to a point ; sixth somite longer than the preceding. Telson nearly as long as the sixth 

 somite. 



First pair of antennae biflagellate. 



Second pair of antennae long, slender, and furnished with a sharp pointed scaphocerite. 



First pair of pereiopoda long, slender, and styliform. Second pair more robust than 

 the first pair and chelate. Three following pairs simple. 



Pleopoda two-branched, terminal pair well developed, foliaceous, rounded at the 

 extremity and subequal with the length of the telson. 



Dana states as one of the characters of the genus that the third somite of the pleon 

 is dorsally produced to a spine. It was so produced in the two species known to him, 

 and such is also the case with those in the Challenger collection, but there is a closely allied 

 specimen that is dorsally nearly smooth, and I hesitate to consider this character as 



