CAMBRO-SILURIAN MICRO-PALEONTOLOGY. 43 



P. hexagonalis differs from all the species of the genus in the super- 

 ficial characters of the zooecia, the sub-angular apertures and rounded 

 interstices bringing to mind some of the monticuliporoids, or perhaps 

 more still, such species of Ptilodictya as P. pavonia, d'Orbigny. Pachy- 

 dictya splendens, Ulrich (Rept. lll.Geol. Sur., vol. 8) from an equivalent 

 horizon in Illinois I regard as a closely related species, differing mainly 

 in having the apertures of the zooecia arranged between fine loniritu- 

 dinal lines. 



Stony Mountain, Manitoba, T. C. Weston, 1881. 



Pachydictya magnipora. (N. Sp.) 

 (Not figured.) 



This species is founded upon a single example collected by Dr. R. 

 Bell, in 1880, at St. Andrews, Manitoba, together with Pachydictya 

 acuta, Hall, Phylloporina Trentonensis, Nicholson, sp., and Monticulipora 

 Wetherbyi, Ulrich. 



At first I thought it might be the same as the P. Everetti, Ulrich, 

 (111. Geol. Sur. Rep't., vol. 8), but on comparing it with the types of 

 that species it proved to have much larger zooecia. The cells are, in 

 fact, larger than in any other species of the genus known to me. 

 P.foliata, Ulrich, from the Trenton shales of Minnesota, is probably 

 its nearest congener. The following brief diagnosis will suffice for the 

 recognition of the species : 



Zoarium, a thin, slightly undulating, bifoliate expansion, attaining 

 dimensions of probably forty or fifty mm. wide and as much or more 

 high. At intervals of about five mm. the surface exhibits seemingly 

 solid smooth spots or maculae. Apertures of the zooecia, oval, large, 

 0*64 mm. long, by 0'4 mm. wide, arranged in regular longitudinal and 

 diagonally intersecting series, with seven or eight in five mm. longi- 

 tudinally, and ten in the same space diagonally. This arrangement is 

 not much disturbed by the maculae. Many of the apertures are closed 

 by a thin calcareous cover, in which I can detect, occasionally, a small 

 central perforation. Interspaces flat or faintly concave, narrow, usually 

 not more than one-third as wide as the apertures of the zooecia. 

 Internally the interspaces and maculae are occupied by shallow 

 vesicles. Median lamina finely striated longitudinally and obscurely 

 wrinkled transversely. The longitudinal striae are very equal and 

 represent the " median tubuli," which constitute such a characteristic 

 feature of the Stictoporidoi. 



