560 PALEONTOLOGY OF ILLINOIS. 



opposite in adjoining rows, on others alternate. Fenestrules 

 oval or subquadrate, about 0.5 by 0.2 mm., with fourteen in 

 1 cm. 



The delicacy of the zoarium is its distinguishing feature. 



Position and locality: Hamilton group, Rock Island, Illinois 

 and Davenport and Buffalo, Iowa. 



HEMITRYPA PROUTANA Ulrich. 



PL LVII, fig. l-lc. 



Fenestella hemilrypa Prout, 1859. Trans. St. Louis Acad. Nat. Sci. Vol. I, p. 444, PL 

 XVII.flg. 4-4a. 



Zoarium a large flabellate or semi-infundibular expansion, 

 more or less undulated toward the free margins. Obverse or 

 inner side protected by a delicate net-work, formed by the union 

 of longitudinal and transverse bars. The former consists of two 

 kinds, a slightly stronger and more prominent set, which are 

 developed directly over the center of the branches and united 

 to them by short supporting pillars. These may be called the 

 principal bars. A second set, which alternates with the prin- 

 cipals and may be known as the secondary bars, are suspended 

 over the space between the branches. The net-work is com- 

 pleted by a set of short transverse bars to which Prof. Hall 

 applies the appropriate term "scalte." According as the scalse 

 are developed oppositely or alternately in the adjoining longi- 

 tudinal series, the principal and secondary bars are straight or 

 zigzag, and the interstices, consequently, may be quadrate, pen- 

 tagonal or hexagonal. In this species the last form is less the 

 rule than usual. Measuring transversely, about twenty-six rows 

 of interstices occur in 5 mm.; longitudinally about twenty-four. 

 One to three small denticles sometimes project from the bars 

 into the interstices. When this net-work has been denuded, 

 which, on account of its delicacy, is often the case, the obverse 

 face of the branches is seen to be ridge-shaped, from 0.2 to 0.32 

 mm. wide, and generally twenty-six in 1 cm. The median carina 

 is neither sharp nor prominent, but carries small nodes (the 

 broken pillars that support the superficial net-work) at intervals 

 corresponding with the length of the zooecia. Dissepiments very 



