American Palaeozoic Bryozoa. 



123 



2. The discoid zoarium, the upper or convex surface of which is 

 occupied by the cell apertures, while the lower surface is covered with 

 a striated and wrinkled epitheca. Good examples of this form are 

 found in Diplotrypa petropolitana, Pander, Monotrypa petasiformis, 

 Nicholson, Prasopora selwyni, Nicholson, and Amplexopora discoidea, 

 James. 



3. The dendroid or ramose zoarium, in which the entire free surface 

 is covered by the cell-apertures. This type may be variously modified, 

 by the flattening of the branches, etc. Of species possessing a ramose 

 zoarium mSy be mentioned, Callopora eleyantula, Hall, C. ramosa, 

 D'Orb., Batostomella gracilis. James, B. tumida, Phillips, Monotry- 

 pella quadrata, Rominger, and Batostoma implicata, Ulrich. 



4. The Jrondescent zoarium, which consists of a widely expanded, 

 and compressed frond, in which the tubes are vertical in the median or 

 axial portion of the expansion, and diverge outwards to open on both 

 surfaces as growth proceeds. This type of growth is represented by 

 Het.erot rypa frondosa, . D'Orbigny (not Nicholson), Trematoporc 

 dfunsoni, Nicholson. 



• 4. The laminar or double leaved zoarium, in which the tubes 

 diverge from a central plane, marked by a thin, but double, calcareous 

 lamina or epitheca. Species possessing this type of growth are well 

 represented by Peronopora decipiens, Rominger. Fistulipora Jin- 

 bellum, Rominger, and Didymopora </ppressa, Ulrich. 



. 6. The encrusting zoarium, which is parasitically attached by the 

 whole of the under surface to foreign bodies. The tubes are short. 

 Of these may be mentioned, Nebulipora papillata, McCo\*, and all the 

 species of Spatiop>ora and Atactopora, Ulrich. 



7. Another type of growth is formed by Monotrypa ? calceola, Miller 

 and Dyer, M, ? clavacoidea, James, and M. ? concava, Ulrich. In 

 these the zoarium is free, and the tubes arise more or less abruptly 

 from a horn-shaped (J/. ? calceola), an elongated conical (J/. ? 

 clavacoidea), or simply concave cavity, which is lined with a thin and 

 finely striated epithecal membrane. 



• II. Surface characters. — There are only two superficial features 

 which I propose to mention under this head, and these are the " monti- 

 cules," and the "maculae." The first of these are circumscribed areas 

 on the surface of the zoarium, which are more or less elevated above 

 the general level, so as to constitute a more or less regularly dis- 

 tributed series of conical, rounded, or elongated eminences. Some- 

 times the moLticuUs are occupied by cells differing in no special feat- 



