44 



three very large uniform spines, the tips of which point forwards, 

 and almost overhang the base of the spine in front. Here the 

 two rows are parallel, but behind the cervical groove they begin 

 gradually to converge, and the spines to diminish in size. There 

 are seven of these in each row behind the groove. 



A somewhat similar arrangement of spines occurs in P. 

 Gilchristi. Mr. Stebbing has verj^ kindly lent me a specimen 

 of this South African species for comparison with the Irish 

 specimen. * I find that the dorsal rows are composed of much 

 larger and stronger spines in the latter than in Mr. Stebbing's 

 specimen ; they are also more uniform in size and regular in 

 arrangement. 



The large supraorbital spines are more horizontal than in 

 either P. vulgaris or P. Gilchristi, and the distance between the 

 tips of these spines is greater than in the other two species. 

 The distance from tip to tip is exactly half the length of the 

 carapace, whereas in the other two species mentioned it is 

 always markedly less than half. On the anterior margin of 

 each of these spines there are four small teeth of uniform size. 

 The posterior margin is smooth. Behind this large spine and 

 also behind the suborbital spine there are diminishing rows 

 of smaller spines, as in P. vulgaris. 



The rest of the spiny armature of the carapace is more robust 

 than in the specimen of P. Gilchristi which I have examined. 



On the front edge of the epistome there are several small 

 tubercles, in addition to the sharp central tooth and those at 

 the external angles ; they are not so large or so sharp as in 

 P. vulgaris. 



The pear-shaped body at the anterior end of the sternum 

 bears two small tubercles. 



The most important difference between this species and P, 

 Gilchristi lies in the sculpture of the abdominal terga. On each 

 of these from the second to the fifth there is a transverse furrow 

 which is interrupted in the median line by a low flat ridge or 

 carina ; but in P. Gilchristi in addition to this, there is also on 

 each of these segments an anterior furrow which is unbroken 

 by the carina and which is heavily fringed with setae. In neither 

 P. Thomsoni nor P. vulgaris is this second furrow present ; 

 there is at most a very faint depression on the second segment 

 alone, and this bears only a few setae. The presence of this 

 important furrow is not mentioned in the original description of 

 P. Gilchristi (Stebbing 1900 {h) ). 



The spines of the abdominal pleura are quite as long and as 

 sharp as in P. vulgaris. There is a sharp tooth on the anterior 

 edge of the pleura of the second segment. 



The first pair of legs are stouter and shorter than the succeeding 

 pairs, but not very markedly so. On the inner side of the ischium 

 there are two blunt tubercles. The inner crest of the merus 



* Since writing the above, I have seen another specimen of P. Gilchristi, kindly 

 sent to me by Dr. Caiman. It agrees exactly with Mr, Stebbing's specimen. 



