ARACHNIDA 



453 



structures have been described which correspond to the pectines in 

 position and structure. If this is substantiated, it constitutes a 

 remarkable resemblance in detail. 



A few special characters may be mentioned here. On the ventral 



Fig- 334- Diagrams of extinct Arachnida. A, Pterygotus osiliensis, dorsal 

 view. After Schmidt. B, Slimonia (restoration of ventral surface by M. 

 Laurie). C, Hemiaspis limuloides, dorsal surface. From Woods. All Silurian 

 forms. Segments and appendages numbered to correspond ; Arabic numerals 

 in Pterygotus and Roman in Slimonia. chc. chelicerae (segment i); b. meta- 

 stoma ; d. compound eyes ; e. simple eyes ; g.op. genital operculum ; tel. telson. 



surface a structure called the metastoma is seen which possibly re- 

 presents the pregenital segment. Branchiae undoubtedly existed, but 

 their exact nature is not known. Possibly the sterna of the segments 

 which carried them were membranous and the branchiae were tucked 



