34 GEOLOGICAL AND NATURAL HISTORY SURVEY OP CANADA. 



more numerous acanthopores. B. irrasum, Ulrich, from the Trenton 

 shales of Minnesota, has larger acanthopores and usually fewer meso- 

 pores, and B. fertile, Ulrich, from the same locality and position, less 

 ring-like and more polygonal zooecia walls. All of these species ditfer 

 also more or less in their vertical sections. B. Girvanense, ITicholson 

 (sp.), appears to agree more closely than any of the others. It also 

 has very small acanthopores, but they are more abundant. In com- 

 paring vertical sections B. Manitohense will be seen to differ in having 

 much less numerous diaphragms in the axial region. It may be noted 

 also that some examples (those having subpolygonal zoa^cia) resemble 

 B. variabile, Ulrich, while the surface of the typical form is much like 

 that of certain species of Trematopora (e: g. T. Halli^ Ulrich, and T. 

 calloporoidea, Ulrich). 



Several small examples are referred here provisionally. These frag- 

 ments are only three or four mm. in diameter, but no conspicuous 

 differences were noticed in thin sections. 



Stony Mountain, Manitoba, Messrs. T. C. Weston and A. McCharles, 

 1884. 



Petigopora, Ulrich, 1882. 



This genus includes small parasitic bryozoa. in which mesopores are 

 absent but acanthopores are well developed. The zooecia are polygonal, 

 diaphragms are very few or absent, and the outer margin of the zoari- 

 um is usually formed by a narrow non-celluliferous band. That the 

 affinities of the various species referred here are with Dekayia wiWf I 

 think, scarcely admit of doubt. They might even be disposed of by 

 regarding them as parasitic species of that genus. Still, as Petigopora 

 complies with the real purpose of classification in being a convenient 

 designation of a natural group of species, the name should be preserved. 

 The group holds the same relation to Dekayiaas Leptotrypaio Amplexo- 

 pora, Monotrypa (pars) to Monotrypella, and Heteroporella to Heteropora. 

 All these groups are not only convenient in classification but natural 

 as well. 



Petigopora scabiosa. (N. Sp.) 



(Not figured.) 



Zoarium parasitic, forming subcircular or irregular crusts upon shells 

 and other foreign bodies. Crusts small, slightly convex, thin and in 



