American Palaeozoic Bryozoa. 



237 



or attached at the base to some foreign substance. Surface smooth, or 

 faintly raised at intervals into low and broad monticules, which are 

 occupied by groups of cells of a larger size than the average; the 

 largest have a diameter of about -^th inch. The ordinary cells have 

 a diameter varying from y^th to -j-g-oth of an inch, are polygonal, and 

 have very thin walls. 



Longitudinal sections (Plate X., fig. 16,) show the tubes in the 

 "immature" portions of their length to have excessively thin walls, 

 and to be crossed by straight, though usually obliquely directed 

 diaphragms, placed at distances apart of one tube diameter or a little 

 less; a few of the tubes in this zone have one side lined with corre- 

 sponding]}' large cystoid diaphragms. In the "mature" zones, the 

 tube-walls are slightly thickened, more cystoid diaphragms are de- 

 veloped, and as well, the straight diaphragms become somewhat crowded. 

 At the junction between the upper end of the "mature," with the 

 lower portion of the next succeeding " immature " zone, the continuit} T 

 of the cell-walls is always more or less disturbed. The young tubes 

 have their lower end divided transversely b} r numerous diaphragms, 

 but they rapidly attain the diameter and character of the more 

 fully developed tubes. 



Transverse sections (Plate X., fig. la) show that all the tubes of the 

 zoarium are polygonal and thin-walled, and the walls, in the mature 

 region, have a peculiar granular appearance, with light streaks apparently 

 passing through the substance of the walls, as though the} f might have 

 been porous.* As is the case in all Monticuliporoid species possessing 

 cystoid diaphragms, the visceral chamber of the tubes is crossed by 

 a delicate lamina, which is deeply excavated on one side in a triangular 

 or crescentic manner, their presence being due to the intersection of the 

 cystoid diaphragms, that, as is shown in a longitudinal section, line 

 one side of the tube. Interspersed among the tubes that have attained 

 the mature size, are a small number of more or less developed young 

 tubes. 



The form to which the attached examples present the greatest re- 

 semblance, and with which it may be readily confounded, is the Praso- 

 pora hospitalis, Nicholson. The external points of difference are found 

 in the slightly larger cells, thicker cell walls, and numerous interstitial 

 cells, characterizing Nicholson's species. In comparing an} T well pre- 



* The same character may be observed in M. mammulata, and other species, anditseems 

 probable that the walls of the tubes, in species of Monticulipora, were pierced by connecting 

 foramina. 



