Tertiary Spiders and Opilionids. 223 



the first abdominal somite. There is therefore a deep constric- 

 tion between the carapace and the abdomen, whereas in Opihones 

 and Acari no constriction is noticeable because the abdomen is 

 broadly joined to the cephalothorax. In Ricinulei the abdomen 

 is also provided with a petiolus, but the coupling" of the abdomen 

 with the carapace is such that it has all appearance of a broadly 

 joined abdomen. Moreover the Ricinulei possess a cucuUus in 

 front of the carapace. The Solifugre have certain similarity with 

 spiders although their abdomen is Ijroadly joined to the carapace. 

 What permits an easy separation of the Solifugie from the spiders 

 is the difference in the structure of their carapace and chelicer?e. 

 The carapace in Solifuga^ represents the fusion of the cephalic 

 tergites with the first thoracic tergite, leaving the second, third and 

 fourth thoracic terg-ites free, whereas in spiders all thoracic ter- 

 gites are fused with the cephalic tergites. The cheliceree of spiders 

 are retrovert, those of Solifug?e chelate. From other orders of 

 Arachnida in which the abdomen is also joined to the carapace by 

 means of a petiolus the spiders can be distinguished by their 

 chelicerae and pedipalpi. In recent forms confusion is impossible. 

 In extinct forms members of the sub-order Amblypygi of the 

 order Pedipalpi could be easily confused with spiders if the preser- 

 vation were deficient. In such cases the relative size and position 

 of the coxffi comes to our aid. The possession of spinnerets on 

 the abdomen is an exclusive characteristic of spiders, and if spin- 

 nerets are preserved spiders can be recognized as such even if all 

 other characters are indisting'uishable or missing. 



Tertiary spiders possess already all the characters of recent 

 spiders, including the epigynum in females and the copulatory 

 apparatus on the terminal joint of the palpus in the male. The 

 external appearance of Tertiary spiders is much the same as in 

 common spiders of the present time and their habits, in a general 

 way, must have been similar. It is difficult to say whether the 

 paucity of material is attributable to the poor conditions prevailing 

 in the Tertiary for the preservation of spiders, or to the relative 

 paticity of the spider fauna itself of that period. For the recent 

 spider fauna runs into several hundred as against the forty-seven 

 Tertiary spiders of the same geographical region. Considering 

 the numerous species preserved in the European amber we may 

 reasonably assume that the real number of American Tertiary 

 spiders must have been considerable. 



