1906-1907.] BritisJi HydrachnidcB : The Genus Ptotia. 375 



shield, one pair on each side. The posterior pairs are much 

 larger and are placed directly behind the others with a wide 

 space between the inner margins. The legs, four on each side, 

 are attached to the epimera. The first, or anterior, pair are 

 the shortest and have the least number of long swimming- 

 hairs. Each pair, going backward, gets longer and more 

 hairy. The fourth, or posterior, pair are very long and 

 strong and well supplied with long swimming- hairs, — the 

 swimming-hairs, so-called, being the long ones found near the 

 joints of the legs. Each leg is composed of six segments 

 — viz., the coxa, articulated to the epimeron, trochanter, 

 femur, patella, tibia and tarsus, — the distal segments being 

 usually thinner than the proximal. The patella and tibia are 

 best equipped with swimming-hairs. All the tarsi are fur- 

 nished with retractile claws. 



The genital area of the females is composed of a long cleft 

 with lips on each side, outside of which there is usually a 

 special plate or plates with numerous acetabula. These 

 plates may be tongue-, disc-, or sickle-shaped, or modifications 

 of these forms, according to the species, as will be seen in the 

 descriptions given later on. Sometimes there is more than 

 one plate on each side, and in two or three cases a number of 

 acetabula are found free on the skin. These acetabula also 

 vary much in size, some species having two or more much 

 larger and more conspicuous than the others. 



Each palpus is composed of five segments, the second being 

 the thickest and the fourth the thinnest. The palpi are placed 

 directly in front of the maxillary shield. On the flexor edge 

 of the fourth segment are two or more hair pegs. The distal 

 end of the fourth segment has also a strong tooth on the inner 

 edge. At the extremity of the fifth segment are three or four 

 small teeth and two or three small hairs. The mandibles are 

 placed under the maxillary shield. The claw of each mandible 

 is marked with what looks like fine striae, but what Dr 

 Wolcott has stated to be a comb of fine hairs. 



The males differ from the females in several respects. The 

 epimeral plates are closer together, the two posterior pairs 

 nearly meeting on the median line, with the genital area 

 close to the posterior margin of the epimera. Instead of a 

 cleft they are more or less provided with a seminal pocket, the 



