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



Pleurogonium minutum, F. E. Beddard (PI. XI. fig. 13). 



Pleurogonium minutum, F. E. Beddard, Froc. Zool. Soc. LonJ., 1886, pt i. p. 103. 



The third new species of this genus was dredged off Tristan da Cunha in 100 to 

 150 fathoms of water ; it is represented by only a single specimen, which, like the other 

 two species, is a female with eggs in the brood cavity. 



The specimen measures about 1 mm. in length. 



The shape of the body is characteristic of the genus, the anterior region of the thoraz 

 comprising the first four segments being broad, and the posterior region narrow in 

 comparison. 



The head is rounded anteriorly and is extremely convex like the rest of the bod} - ; 

 behind the articulation of the antennae the lateral margin of the head is prolonged into 

 a short process curved forwards. The antero-posterior diameter of the first four 

 thoracic segments is subequal, and the breadth gradually increases up to the third, which 

 is the widest ; the lateral margins of these segments are rounded and they are furnished 

 with short epimera, which are also rounded, and not prolonged into spine-like processes 

 as are the epimera of the two last species, Pleurogonium albidum and Pleurogonium 

 serratum. The dorsal surface of the body is quite smooth. 



The three posterior segments of the thorax have the usual form that is characteristic 

 of the genus ; they are furnished with epimera which are evident on a dorsal view, those 

 of the anterior segments being concealed when viewed from above ; the epimera are 

 truncated at their free extremity and are not prolonged into spines. Between the last 

 segment of the thorax and the abdominal shield there is a short intercalated segment 

 as in other species of this genus. 



The abdominal shield is oval, tapering towards its extremity; at the end of the 

 second third is a notch which marks the articulation of the terminal pair of abdominal 

 limbs; the lateral margin of the abdominal shield for a short space anteriorly to tlii- 

 notch is serrate, but the serrations are not anything like so conspicuous as in the 

 species which I have named Pleurogonium serratum. 



The antennules are practically identical in structure with those of the last two 

 species ; the first joint extends a very little way beyond the lateral process of the 

 head. 



The antenna} are not so much longer than the antennules as tiny air in 

 Pleurogonium albidum, where the proportion in length was as 2:1; in Pleurogonium 

 minutum the antennas are not half again as long as the antennules, the proportion being 

 about 6 : 4. 



In this respect, therefore, the present species is more typical than Pleurogonium 

 albidum, as it is stated by Sars in his definition of the genus that the two pairs of 

 antennary appendages are of about equal length. 



