34 W. M. TATTERSALL. 



Endopoda of the third thoracic limbs (Fig. 10) with the merus longer than the 

 tarsus ; latter composed of six joints ; nail well developed and longer than the last 

 joint of the tarsus. 



Endopods of the remaining thoracic limbs become successively longer and more 

 slender from the fourth to the eighth ; the increase in length takes place chiefly in 

 the ischial joint ; the number of joints in the tarsus of the endopods also increases 

 in the more posterior limbs ; in one specimen there were six joints in the tarsus of 

 the third limb, six in the tarsus of the fourth, ten in the tarsus of the seventh, and 

 twelve in the tarsus of the last limb. 



Genital appendix (Fig. 11) on the last thoracic limb of the male exceedingly 

 long and slender, equal in length to the first three joints of the limb to which it is 

 attached. 



Pleopods (Fig. 12) similar in both sexes, consisting of a single ramus bearing 

 proximally and externally a rather large process tipped with setae. 



Telson (Fig. 13) a little longer than the last segment of the pleon, and more 

 than twice as long as broad at its base ; cleft at the apex for nearly a quarter of its 

 length, cleft rather wide, its margins armed with about eighteen teeth on each side ; 

 the apex of each lobe of the cleft armed with a pair of spines, the inner one the 

 shorter ; lateral margins armed throughout their whole length with about seventy 

 spines, which become arranged in series towards the apex. 



Inner uropods slightly longer than the telson, with a row of moderately slender 

 and long spines on its inner ventral margin, varying in number from twenty-six to 

 twenty-eight, and extending from the otocyst to near the apex ; spines not arranged in 

 series, but increasing in size distally. In some specimens the spines extend further 

 down the uropod than in others. 



Outer uropods about half as long again as the inner. 



Length of an adult female with embryos in the brood pouch, 21 mm. ; of an 

 apparently adult male, 25 mm. A second female with embryos in the brood pouch 

 measured 23 mm. 



It is not a little interesting that this genus should have been discovered almost 

 simultaneously in the northern and southern hemispheres, M. farrani, Holt and 

 Tattersall (1906 (2)) having just been described when the 'Discovery' collections 

 came to hand. 



M. posthon is a more stoutly built form than M. farrani, and is further 

 distinguished from the latter in the following characters : 



Antennules. M. farrani has not the outer corner of the basal joint of the 

 peduncle produced nearly as much as in M. posthon. 



Thoracic limbs. The tarsus of the endopods in M. posthon is composed of six to 

 twelve joints, while in M. farrani there are only four. 



Genital appendix to the last thoracic limb of the male is much longer and more 

 slender in M. posthon than in M. farrani. 



