American Palaeozoic Bryozoa. 



241 



walls are excessively thin until they reach the peripheral regions, 

 when they are much thickened, and bend outward to open at the 

 surface. In the peripheral or "mature" portion of the zoarium, the 

 tubes are provided with a series of cystoid diaphragms ; the space in- 

 tervening between their flexuous inner line, and the opposite wall of a 

 a tube, is crossed by equally numerous straight diaphragms. The 

 tube-walls are perforated by rather large connecting foramina. In the 

 tuberculated species the spiniform tubuli are numerous, but very small, 

 and not easily recognized, while in the smooth forms they are much 

 larger, "and constitute a conspicuous feature in sections. The internal 

 structure of the small tubes, which form the maculae of some species, 

 is not remarkably different from that of the ordinary tubes. The only 

 difference that I have been able to detect is found in the fact that cys- 

 toid diaphragms are but rarely developed in them. 

 Type : Homotrypa curvata, n. sp. 



By comparing the above description with my erroneous definition of 

 the subgenus Trematopora, on page 3 53, it becomes apparent that both 

 were based upon the same group of species. As before stated, I was 

 formerly in doubt whether they could be separated generically from 

 Monticulipora, but now I do not hesitate to say that they are entitled 

 to rank as a distinct genus. The zoaria of all the species of Monti- 

 culipora are, normally, incrusting or massive, while in Homotrypa 

 they are truly ramose or frondescent, and the difference between the 

 characters of the tubes in the axial and peripheral regions of the 

 zoarium, is always strongly marked and constant. As is shown in 

 tangential sections, the tubes in the "mature" region of a species of 

 Homotrypa, have thick walls, and the visceral cavity is more or less 

 rounded, and not polygonal, while it can not be said of any species of 

 Monticulipora that it has thick-walled cells, or that the visceral cav- 

 ities of the tubes are not polygonal. Internally, Trematopora tuber- 

 culosa, Hall (the type of that genus), differs from species of Homo- 

 trypa in having peculiarly inflected, thin-walled tubes, which are sur- 

 rounded, and otten complete^ isolated by smaller, angular, and closely 

 tabulated interstitial tubes. External ly, the proper cells differ in having 

 their margin raised into a thin rim, which, however, seldom extends in 

 a continuous line around the cell-aperture. 



Beside the two species next described, the Cincinnati Group furnishes 

 at least three other distinct forms, having the characters of this genus. 

 One of these was described and figured by Nicholson, under the name of 

 Monticulipora (Heterotrypa), dawsoni (" Genus Monticulipora,'' p. 



