loz AMERICAN SPIDERS 



plates. So familiar is their outline that we immediately associate 

 them with spiders visible about us, and discern a close resemblance 

 to the liphistiids and the trap-door spiders. During the same era, 

 this early mygalomorph was also found in Europe, more numerous 

 in species and so much more diversified that the imprints seem to 

 belong to several distinct types. All have well-marked tergites. 



The Illinois spiders from the Pennsylvanian shales of Mazon 

 Creek are placed in the genus Arthrolycosa, and in the family 

 Arthrolycosidae, and they are remarkably like the modern species 

 of the family Liphistiidae. However, nothing is known of their 

 spinnerets, claws, sternum, or of other features largely used in clas- 

 sification. After a brief glimpse of them in the coal measures of the 

 northern hemisphere, we lose sight of them completely and can 

 only speculate on their subsequent history. 



From creatures like Arthrolycosa and its European cousins has 

 been developed all the assemblage of modern spiders known as 

 liphistiids, trap-door spiders, funnel-web tarantulas, and typical ta- 

 rantulas; in short, all of the My galomorphae in the broad sense. If 

 we agree with Eugene Simon, the master arachnologist, that the 

 liphistiids are only primitive members of the mygalomorph spiders, 

 we have no difficulty in accounting for the restricted, more typical 

 recent members. The insistence of many specialists that the myga- 

 lomorph spider of the Paleozoic completely lacked dorsal segmen- 

 tation of the abdomen is unreasonable. The tarantulas are present 

 in the Paleozoic with plates on the back of the abdomen; and many 

 of them have retained well-marked evidences of dorsal segmentation 

 through three or four hundred million years until the present time. 



THE TRUE SPIDERS 



Certain shadowy forms from the Carboniferous Era, contempo- 

 raries of the oldest tarantulas, have been assigned with some con- 

 fidence to the Araneomorphae, or true spiders. They appear to lack 

 hard plates on the abdomen, and to assume in a vague way at least 

 the form of some of the highest spiders. In what ways do these 

 emergent creatures, from which is derived the vast array of mod- 

 ern true spiders, differ from the Paleozoic tarantulas? How did the 

 branches separate? 



The fundamental change may well have been one of behavior, a 

 change in habit or attitude rather than a physical alteration. In some 



