ANCESTRAL SPIDERS AND THEIR HABITS. 447 



The discovery of fossil spiders on this continent is confined chiefly to 

 a single point, Florissant, Colorado, although Green River, Wyoming, and 



Quesnel, British Columbia, have contributed some specimens. 

 <.,., The remains occur in a series of lacustrine deposits formed 



within an ancient lake basin which lies in the valley of the 

 present South Fork of Twin Creek, and of the upper half of the same 

 after the South Fork has joined it. This ancient Florissant Lake basin 

 lies among a series of low wooded hills and ravines marked by an irreg- 

 ular L-shaped grassy meadow. At the period of the Oligocene this ele- 

 vated lake must have been a beautiful shallow sheet of fresh water. It 

 was hemmed in on all sides by granitic hills, whose wooded slopes came 

 to the water's edge, sometimes, especially on the wooded sides, rising 

 abruptly, at others gradually sloping, so that reeds and flags grew in the 

 shallow waters by the shore. The waters of the lake penetrated in deep 

 inlets between the hills, giving it a varied and tortuous outline. Steep 

 promontories projected abruptly into the lake from either side, dividing 

 it into a chain of three or four unequal and irregular ponds united by a 

 narrow channel to a larger and less indented sheet, dotted with numerous 

 long and narrow wooded islets just rising above the surface. Along these 

 wooded islands and indented shores, a most congenial habitat, the spiders 

 of the Tertiary had their homes. The Orbweavers and other Sedentary 

 groups hung their snares among the branches of young hickories, oaks, 

 birches, poplars, willows, elms, wild roses, sumac, alder, ferns, catalpa, and 

 bignonia, precisely as in our own woods; or spread them among the blos- 

 soms of water lilies and clumps of grasses, reeds, and iris that thrust their 

 stalks out of the shallow waters, as one may see to-day in the ponds of 

 New Jersey and the lagoons of the South. 1 



The promontories projecting into this lake bed on either side are 

 formed of trachite or other volcanic lavas ; masses of the same occur at 



many different points along the ancient shore. They seem to 

 Cause of ^ Q confined to the edges, for the most part, but some of the 



mesas, or ancient islands, have trachite flows over them, and 

 merit. 



their slopes covered with quantities of vesicular scoriae. We 



have thus pointed out the principal cause of the fossil strata whose ex- 

 ploration has uncovered for us these pages in the life of the spiders of 

 the Tertiary. The shales of the lake in which the myriad of plants and 

 insects are entombed are wholly composed of volcanic ash and sand, which 

 lie fifteen feet thick or more in alternating layers of coarser and finer 

 material. 2 



1 Lesquereux identifies these as among the plants found in the fossil yielding strata. 

 The genera are identical with the corresponding existing plants. U. S. Geolog. Surv. Terr., 

 Vol. VII., Tertiary Flora, 1878. Insects and spiders are usually found in the same shales 

 that yield the plants. 



2 Paleontology of Florissant, S. H. Scudder. 



