240 R. I. POCOCK. 



What is true of tlie Pedipalpi and Aranete is also true of 

 the Xiphosura3. In Limnlus there is a relatively wide sternal 

 area extending from the mouth to the posterior extremity of 

 the prosoma, and strengthened by a pair of strong meta- 

 sternal sclerites behind and a weakly chitinised promeso- 

 sternal plate in front. 



Correlated with this primitive development of the sternal 

 area we should expect to find entosternites of a more archaic 

 type in the Pedipalpi, Araneas, and Xiphosuras than in the 

 other orders of Arachnids. This expectation is justified by 

 the unmistakably metameric arrangement of the constituent 

 elements of the entosternite exhibited in these three orders. 

 On almost any theory of the origin of this plate, segmental 

 repetition of its parts must be postulated as a primitive 

 feature. It is obvious that this characteristic is manifested 

 in a far greater degree in the entosternite of the three orders 

 named than in that of the Scorpiones, Pseudo-scorpiones, or 

 Opiliones. 



A satisfactory settlement of the homologies of the several 

 parts of the various types of entosternites already described is 

 a matter of no little difficulty on account of the variation in 

 number of the apophyses that arise from them. In the 

 Aranea) the dorsal apophyses range in number from four to 

 six. In the Phrynidae there are six; in the Thelyphonidae 

 five, with indications of an additional pair on the posterior 

 lobe of the entosternite. 



In the Araneae the apophyses in question are referable to 

 two categories, those that are directed obliquely inwards 

 towards the central apodeme of the carapace, and those that 

 arise subvertically to be inserted serially along the area 

 between its middle line and lateral border. The latter 

 are invariably present, and invariably four in number on each 

 side ; the former are either absent or rej)reseiited by one or 

 two pairs. In the two types of entosternite ])i-esented by the 

 Pedipalpi there is a single ]iair of apo])hyses directed 

 inwards and backwards, arising in each case close to the 

 base of the anterior cornua. 



