creditensis, and in the lower part of the Streetsville member. The character of 

 the surface is like that of the type in being somewhat variable; in some forms it 

 is quite smooth and in others it has low monticules scattered over it. The groups 

 of large-sized tubes, often seen in the type species, are apparently wanting in 

 the Credit River specimens. The peculiar tabulation of the zooecial tubes and 

 mesopores so characteristic of H. subnodosa is very clearly seen in vertical 

 sections. H. subnodosa ranges from the Lower Arnheim to the Elkhorn in 

 Ohio, and, therefore, is of little use in the determination of the more detailed 

 geological subdivisions. 



Heterotrypa definita, sp. nov. — See page 21. 



Heterotrypa prolifica, Ulrich. — This species occurs in the bryozoan reef in 

 the Strophomena varsensis zone of the Erindale member. It differs from the 

 type species in a few minor details. The acanthopores in the Credit River forms 

 are a little larger than in the type and show a slight variation in size. There is 

 also a greater tendency toward the grouping of mesopores in the type species. 

 In Ohio, according to Shideler, H. prolifica ranges from the Lower Arnheim to 

 the Upper Whitewater and, accordingly, is of little use for purposes of exact 

 correlation. See Plate V, Figure 3. 



Heterotrypa robusta, sp. nov. — See page 22. 



Heterotrypa simplex, sp. nov. — See page 22. 



Heterotrypa simplex maculosa, var nov. — See page 23. 



Heterotrypa cf. subfrondosa (Cumings). — A large frondescent form with 

 indistinct monticules is common in the bryozoan reef in the Strophomena varsensis 

 zone of the Erindale member. In general appearance, it is like H. prolifica, 

 but differs surficially in being smoother and internally in the possession of more 

 numerous mesopores and a much more distinct variation in size of the acantho- 

 pores. It closely resembles H. subfrondosa (Cumings), of the Fairmount of 

 the Ohio valley. 



Heterotrypa subpulchella parvulipora, var nov. — See page 23. 



Homotrypa communis, Bassler. — This species is found in association with 

 H. streetsvillensis in the Streetsville member. . The two species cannot be dis- 

 tinguished from each other until tangential sections, prepared from each, are 

 examined. The tangential section of H. communis reveals the presence of 

 maculae, consisting of secondarily calcified mesopores, which are lacking in 

 H. streetsvillensis. Of the two species H. streetsvillensis is much more abundant 

 on the Credit river. According to Shideler, H. communis is found in the 

 Upper Waynesville and Lower Liberty of Ohio. 



Homotrypa creditensis, sp. nov. — See page 24. 



Homotrypa cf. richmondensis , Bassler. — A fourth species of Homotrypa 

 occurs in the Homotrypa streetsvillensis zone, but in such a poor state of preser- 

 vation that the species can not be determined with any degree of satisfaction. 

 The character of the growth can not be ascertained, but the features shown 

 by tangential sections are like those of H. richmondensis of the Whitewater of 

 the Ohio valley. 



Homotrypa streetsvillensis, sp. nov. — See page 24. 



