610 PALEONTOLOGY OF ILLINOIS. 



surfaces moderately convex, the width of a branch being greater 

 than its thickness. When well preserved the reverse side is 

 finely striated. 



This is one of the most characteristic species of the Chester 

 group, being moderately abundant at all of the typical locali- 

 ties where the horizon is exposed. It is also very distinct from 

 all the American forms, the principal peculiarities being the four 

 ranges of zooecia and the slender branches. A similar species is 

 described by Mr. G. R. Vine from the carboniferous shales of 

 England, as T. gracilis. 



Position and locality: Chester group, Chester, Kaskaskia, and 

 other localities in Illinois, and Sloan's Valley and Litchfield in 

 Kentucky. 



THAMNISCUS RAMULOSUS Ulrich. 



PI. LXII, fig. 4-4b. 



Zoarium a small frond, composed of delicate, frequently bifur- 

 cating branches, the bifurcations occurring at intervals of 1 or 

 2 mm. Branches rounded, from 0.3 to 0.5 mm. wide, two- 

 thirds as thick, connected to each other by slender dissepiments, 

 which never outnumber the bifurcations. Zooecia increasing 

 from three to five ranges between the bifurcations. Apertures 

 small, pustuliform, about 0.08 mm. in diameter, several times 

 their diameter apart, not very regularly arranged in oblique 

 transverse rows, with twelve in 3 mm. A small tubercle may 

 now and then be detected. Reverse with five or more longitu- 

 dinal striae, which are less distinct near the base of the frond. 



var. SEVILLENSIS Ulrich. 



PI. LV, fie. 6, and PL LXH, figs. 5-5a. 



In this variety the zoarium spreads more rapidly than in the 

 typical form. The zooecia and their apertures are somewhat 

 larger, there being only nine or ten of them in 3 mm., while 

 there are also never more than four, and usually only three 



