10 SCOEPIO>*ES. 



aculeus pointing backward. The anus is placed beneath the 

 vesicle, at the end of the 5th tail-segment ; the edge of the latter 

 adjacent to the anus is termed the anal border. All the caudal or 

 tail-segments are as a rule carinate, crested or keeled at the sides 

 and below ; the keels, when fully developed, being eight or ten in 

 number on the first four segments, whereas the 5th has only five 

 complete keels, one of which occupies the middle line of its lower 

 side. From above downward on each side the keels of the first 

 four segments bear the following names superior, superior lateral, 

 median lateral, inferior lateral, inferior median. The keels of the 

 5th caudal segment are named superior lateral, inferior lateral, and 

 inferior median (see fig. 8, p. 17). 



Although the tail is in reality part of the abdomen, the latter 

 term is, for descriptive purposes, restricted to the seven somites 

 that precede the tail. The last abdominal somite is narrowed 

 posteriorly, and is usually crested both on its tergum and sternum 

 after the manner of the tail-segments, each being furnished with 

 four keels. Each of the four sternal plates that precede the last 

 is provided with a pair of elongate or circular respiratory stigmata , 

 on the inner side of the stigma there is a distinct groove, the 

 muscular impression. The sternum of the 2nd abdominal somite 

 is very small, but bears a pair of comb-like appendages (the pectines). 

 These consist of a handle or shaft and a series of movable teetJi. 

 Between the bases of the teeth there are small rounded skeletal 

 pieces called the fulcra. The shaft is usually segmented. Between 

 the sternum that bears the pectines and that of the cephalothorax, 

 already described, is situated the generative orifice : this is 

 covered by a single or double movable lid, the genital operculum. 



Measurements. In the specific descriptions the measurements- 

 are taken as follows : Total length is taken from the anterior 

 border of the carapace, not including mandibles, to the tip of the 

 aculeus when the vesicle is depressed so as to lie in the same line 

 as the caudal segments ; the carapace is measured from the side of 

 its anterior border to a corresponding spot on the posterior border, 

 and not along the middle line. The tail-segments ai'e measured 

 laterally along the carinate portion, the measurements not includ- 

 ing the short unkeeled cylindrical portion situated at the anterior 

 extremity of the segments, and frequently covered by the posterior 

 extremity of the segment in front. Humerus and brachium of 

 chela are measured along the upper surface ; the underhand along 

 the keel that borders it above or externally, and the movable digit 

 along its outer edge. 



Sexual differences. The male is, as a rule, a little smaller than 

 the female and has a narrower abdomen. Moreover the chelae are 

 frequently longer and the hand is either broader or narrower ; 

 when broader, the fingers are, as a rule, separated at the base, the 

 movable being furnished with a lobe-like tooth which fits into a 

 corresponding notch on the immovable. The tail is generally 

 longer, sometimes very much longer, less commonly stouter at its 

 posterior end ; the vesicle mav be longer and thinner, or more 

 globular or otherwise modified. The pectinal teeth are either 



