412 PALEONTOLOGY OF ILLINOIS. 



HOMOTRYPELLA Ulrich, 1886. 



(14th Ann. Eep. State Geol. Minn. p. Si.) 

 (For generic diagnois see page 370). 



This genus was established to receive a small but natural 

 group of species that could not be included in any of the other 

 genera of the family. The ramose habit of growth ally the 

 genus to Homotrypa Ulrich, but the abundant mesopores are 

 a distinguishing mark of some importance. A comparison with 

 Peronopora Nicholson, and Atactoporella Ulrich, shows the fol- 

 lowing differences: the zoaria of the first are bifoliate, of the 

 latter parasitic; in both the cystiphragms are developed in an 

 almost uninterrupted series to the zooecial apertures. In the 

 structure of the walls the typical species presents close resem- 

 blance to Peronopora, while that of H. context a, shows greater 

 likeness to Atactoporella. The aggregate of characters thus in- 

 dicate an intermediate position between Homotrypa on the 

 one side, and Peronopora and Atactoporella on the other. 



HOMOTRYPELLA CONTEXTA Ulrich. 



PI. XXXII. flg. 5-5b. 



Zoarium consisting of a mass of irregular, coalescing branches 

 varying in thickness from three to six mm. Surface smooth, 

 exhibiting spaces where both zooecia and mesopores are larger 

 than ordinary. Walls of zooecia scarcely thickened in the cor- 

 tical region. Zooecia sub-circular or oval, from eight to ten in 

 two mm., about 0.15 mm. in diameter, occasionally in contact, 

 ordinarily separated by the angular, thin-walled mesopores, 

 which are very numerous and differ but little in size from the 

 zooecia. Diaphragms few in the axial region, but becoming- 

 more numerous as the zooecial tubes approach the peripheral 

 region. Here they are abundant, extending from the overlap- 

 ping cystiphragms to the opposite wall of the tube. In the 

 mesopores the diaphragms are closely set, and approximately 

 upon the same level in all, two or three in the space of a tube 

 diameter. Acanthopores rather small, but a conspicuous feature 



