STUDIES ON THK ARACHNID ENTOSTEKNITI*:. 235 



surface of the stenmm as the so-caHed sigilhi. In many 

 genera there is a tendency for the posterior pair to increase in 

 size and shift their point of insertion i'rom a snbrnnrginal to a 

 snbcentral position. This is particnhirly noticea])le in the 

 so-called "trap-door" spiders^ where the innscles and append- 

 ages of the prosonia are specialised for fossorial work. 



In a few genera, e. g. Atypus, Eriodon, and Actinopns, 

 all the fonr pairs of ventral apophyses have moved from the 

 margin of the sternum towards its centre, the convei'gence 

 reaching an extreme in Actinopus, where their points of 

 attachment meet in the middle line, forming the well-known 

 i-osette or star-shaped sternal impression characteristic of 

 this genus. The union of these four apophyses on each 

 side with one another and with their fellows of the opposite 

 side results in the formation of a solid plate beneath the 

 nerve mass, which is thus enclosed, as it were, in a basket, 

 the lateral nerves to the limbs passing out through the 

 spaces between the npi-ight portion of the apophyses. 

 From the middle of the anterior border of this ventral 

 plate a short median process runs forward, forming the 

 median unpaired lobe of the rosette-like impression on the 

 outer side of the sternum. 



In Atypus the four apophyses retain their primitive dis- 

 tinctness, and are arranged on the underside of the 

 entosternite in the form of a circle, following the curvature 

 of the pharyngeal notch. A fibrous strand luiis forward 

 from the anterior apophysis to the presternum . 



A reduction in the number of ventral apophyses takes place 

 in the typical genera of the Ctenizinfe, e. g. Pachy- 

 lomerus, Stasimopus, and of the Idiopinae, e. g. 

 Acanthodon and Heligmomerus. In Pachylomerus 

 the first and fourth apophyses persist, the second and third 

 disappear. Stasimopus resembles Pachylomerus in this 

 particular, but differs in that the apophyses of the anterior 

 pair fuse across the middle line to form a complete collar 

 round the nerve mass. In Acanthodon the first apo- 

 physis is retained as in all the Mygalomorphas, and the second 



