350 PALEONTOLOGY OF ILLINOIS. 



With the single exception of Thamniscus, in which the 

 branches are usually free, the zoaria of this family form reticu- 

 lated expansions of which only one side is celluliferous, the 

 other being finely granulo-striate. Commonly, the branches are 

 more or less rigid and united to each other by solid processes, 

 which are developed at such regular intervals that their num- 

 ber in a given space affords a good test in the discrimination 

 of species. In other cases the branches are sinuous and united 

 by anastomosis. The fenestrules vary from circular to acutely 

 elliptical, but when the dissepiments are thin, a quadrate form 

 prevails. The primitive cell, or true home of the polyp, is either 

 oblong quadrate or hexagonal, or rhomboidal, and never com- 

 mences, as claimed by Waagen and Pichl, (Pal. Indica, ser. 

 XIII, Salt Range Foss. p. 772) "in a long thin point." On the 

 contrary, in all cases when the cell is not of rhombic form, the 

 proximal end is truncated, being, furthermore, vertically parallel 

 with the anterior wall. As seen in longitudinal sections, the 

 cell is directed more or less obliquely upward and forward, the 

 angle of divergence from the basal plate varying between the 

 extremes of 45 and 90. Generally the height of the cell is 

 considerably less than its length, yet in a few instances the two 

 dimensions are almost equal. The inferior hemiseptum is often 

 present, and springs (apparently always) from the anterior 

 wall near its base. Sometimes it is so strong that in deep 

 tangential sections (PI. LV, fig. 7) the cell appears as though 

 divided into two apartments. 



Another peculiarity that may be seen in a tangential section 

 of almost any Fenestellid (see figures on PL LIV and LV), is 

 due to the presence of a superior hemiseptum. Namely, at op- 

 posite points on the sides of a cell through which the sect on 

 passes, just beneath the level of the primary orifice, the wall is 

 more or less sharply bent in, the inflections being in some cases 

 sufficiently extended to unite, so as to separate an anterior 

 subcircular space from the remaining posterior portion of the 

 cell. Generally, however, the inflection appears strongest on 

 the upper side (especially in Fenestella) giving to the cell a 

 rather marked kidney-form outline (see PI. LI, fig. 5a, and PI. 

 LIV, fig. 5). This appearance is not due to a greater develop- 

 ment of the internal ridge on the outer side, but to the obliquity 



