,74 DISCOVERY REPORTS 



pereion somites in Arcturides, whereas in Psciididothea they are developed as not very distinct rings 

 around the basis of the second to fifth pereiopods. 



Hale's third point requires some amplification. In the species of both Pseudidothea and Arcturides, 

 the coxal development of the first four pairs of pereiopods is exactly similar, the coxal joints of the 

 second, third and fourth pereiopods can be seen in a lateral view of the animal as small rings which, on 

 their inner sides, extend as flat coxal plates over the ventral surface of their respective somites as far 

 as the mid-ventral line. The coxal joint of the first pereiopod is fused with tergum, but its coxal plate 

 does not fuse with that of its fellow in the mid-ventral line. 



In the species of Arcturides the coxae of the last three pairs of pereiopods are fused with the terga 

 of their respective somites, but the suture lines are visible; ventrally the coxal plates of each pair are 

 fused in the mid-ventral line. In the species of Pseudidothea the condition has gone a stage further 

 and the suture lines between the coxae and terga are lost and fusion is complete. In neither genus are 

 these posterior coxal joints separate units in contrast to the second, third and fourth pairs of limbs. 



In Ho/idotea, the remaining genus of the family, according to Barnard (1920, p. 381) the 'side 

 plates ' (presumably coxal in origin) are distinct on all except the first segment ; thus, the three genera 

 form a graded series in which the coxae of the last three pairs of pereiopods are free in Holidotea, 

 fused to their respective terga but with suture lines distinct in Arcturides and completely fused in 

 Pseudidothea. 



A further character which is common to the three genera is the form of the first pleopod in the male, 

 the modification being similar to that met with in members of the Astacillidae, except that the distal 

 portion of the exopodite is curved outwards as a tapering projection (see figures in descriptions of 

 species). This is less pronounced in Holidotea where the endopodite also is reduced in size. In all 

 three the penis is single but bilobed. 



Genus Pseudidothea Ohlin, 1901 



The description of the coxal joint is unsatisfactory in the diagnosis of this genus. Of the species 

 P. bonnieri, Ohlin (1901, p. 276) said: 'epimera of all segments visible from above, being small, 

 slightly triangular tubercles ' ; in his description of the appendages he makes no mention of the first 

 joint. Nordenstam (1933, p. 113) also gave a diagnosis of the genus, and with reference to this point 

 says : ' Coxae developed as incomplete not very distinct rings around the proximal ends of the basi- 

 podites of the second to seventh pereiopods.' 



The coxal joints of the thoracic limbs of the third, fourth and fifth somites (second to fourth 

 pereiopods) can be seen in a lateral view of the animal, as small rings, which extend as flat coxal plates 

 over the ventral surface of their respective somites up to the mid-ventral line. The coxal joint of the 

 second thoracic limb (first pereiopod) is partially fused with the tergum of its segment, the plate is 

 well developed and extends towards the mid-ventral line where it meets its fellow of the other side. 



The coxal joints of the sixth, seventh and eighth pairs of thoracic limbs (5-7 pereiopods) are 

 entirely fused with the lateral parts of the terga of their respective somites. From analogy with other 

 members of the Valvifera it may be assumed that the ventral coxal expansions have fused with one 

 another in the mid-ventral line ; this can be correlated with the fact that the posterior part of the body 

 is narrow and more or less cylindrical in shape. 



As already noted, the penis in the male is enclosed by the first pleon segment (Text-fig. i^); it is 

 single, but its double origin may be seen in its bilobed extremity. 



