356 PALAEONTOLOGY OF ILLINOIS. 



cell mouths. In Isotrypa Hall we have the same general con- 

 struction as in Unitrypa, but the external aspect of the two 

 expansions are so much alike, that it is sometimes difficult to 

 distinguish between them. Indeed, the summits of the keels and 

 the connecting bars are rounded and longitudinally striated, so 

 that they might readily be mistaken for the reverse side of an 

 ordinary species of Fenestella. Sections of a species of this 

 genus are figured on plate LIV. 



In the much disputed Hemitrypa, we find a somewhat differ- 

 ent construction. In this genus the carina (which in Semico- 

 scinium, Unitrypa and Isotrypa is a continuous plate) carries a 

 close series of small pillars that support the favose super- 

 structure. A regular series of arches connect the pillars and 

 combine to form a longitudinal crest at intervals corresponding 

 to the number of zocecia. Short transverse bars proceed from 

 each side of this crest and extend to the center of the space 

 between two branches where they unite with a false crest which 

 is usually somewhat thinner and more decidedly zigzag than 

 the true crest. This arrangement of the bars and crests pro- 

 duces a delicate net- work, pierced by small and exceedingly 

 regular hexagonal, petaloid, or circular openings, This brief 

 description of the structure of Hemitrypa may be advantage- 

 ously supplemented by a study of the sections figured on plates 

 LIV and LVII. None of these are in any way diagrammatic 

 but are drawn as near to nature as possible. 



The free condition of the branches in species of Thamniscus in- 

 duced King to establish another family for their reception.* In 

 this course he is followed by Vinef and by Waagen and Pichl$. 

 Zittel, however, places the genus as a synonym under Acantho- 

 cladia King, being under the erroneous impression that the 

 two genera are distinguished only by differences in the arrange- 

 ment of the zooecia apertures. All of these authors agree in 



* Mono. Perm. Foss. (Pal. Soc. Pub. vol. Ill, 1849.) 



t 4th Kept. Brit. Assoc. on Foss. Poly. 1883 and Notes on Joredale Poly, of N. Lan- 

 cashire, 1885. 



+ Palseontologica Indica, ser. XIII, part 5, 1885. 

 Handbueh der Pal. 1880. 



