SCHIZOPODA, STOMATOPODA, AND NON-ANTARCTIC ISOPODA. I !>:; 



specimens taken at Tristan il'Acunlia. The female specimens agree with LEACH'S 

 brief description in its main points body smooth, terminal segment of the abdomen 

 terminating in an obtuse point and having at its base two elongated and rather 

 indistinct tubercles. The description of the terminal segment of the abdomen and 

 the uropods may be amplified somewhat. The last segment of the abdomen is 

 triangular in shape, narrowing to a rather produced apex, the actual tip of which is 

 bluntly rounded. The segment is not evenly vaulted from its edges, as, for instance, 

 it is in E. yiyas ; but some little way in from the. margins there is a shallow impressed 

 groove running more or less parallel with the margins all the way round. The central 

 portion thus marked off is evenly vaulted, and bears anteriorly two elongated but 

 only slightly pronounced tubercles. The inner and outer uropods are about equal in 

 length, and barely reach the apex of the abdomen. The inner one is truncate at its 

 distal extremity, the outer one evenly rounded. 



The male specimen, 10 mm. in length, which I refer to this species, differs from 

 the female in having the seventh segment of the thorax produced into a short, blunt 

 median process, which projects slightly beyond the anterior margin of the last seg- 

 ment of the pleon (Plate, fig. 1). Moreover, the tubercles on the latter are very 

 obscure and almost obsolete. But otherwise the agreement with the female speci- 

 mens is very close, especially in the form of the pleon and uropods, as described 

 above, though the latter are, perhaps, a little more fully developed. I have very 

 little doubt that the male specimen should be referred to the same species as the 

 females, and I think I am correct in regarding both as examples of LEACH'S species. 



If my identification is correct, the generic position of the species requires con- 

 sideration. As regards the mouth organs and the structure of the pleopods, the 

 specimens are in complete harmony with the genus Exosphseroma. Moreover, they 

 show the closest agreement with the type of that genus, E. yiyas, Leach, in the 

 general form and the structure of the various appendages. They differ from E. gigas 

 in the form of the pleon and uropods in both sexes, and in the process from the 

 seventh thoracic segment in the male. HANSEN'S amended definition (1905)ofthe 

 genus Exosphseroma, however, runs as follows : " Last thoracic segment unarmed in 

 both sexes. End of the abdomen at most somewhat produced, but not acute." If 

 this definition be accepted, LEACH'S species would be excluded from the genus 

 Exosphferoma by the characters of the last thoracic segment of the male, and 

 would fall into one or other of the genera Zuzara and Isocladus. These latter 

 genera are, however, further characterised by the great development of the uropods 

 in the male, greater in Zuzara than in Isocladus, but much greater in both than in 

 the present species, in which the difference between the sexes in this respect is almost 

 negligible. The females of all three genera are very much alike, and UANSEN himself 

 has indicated the great difficulty of separating the genera in a satisfactory manner. 

 Moreover, as STEBBING (1910) points out, he has at least implied a modification of 

 the definition of Exosplui'roma, quoted above, by including in the genus Sphaeroma 



(BOY. SOC. KDIN. TRANS., VOL. XLIX., 883.) 



