310 PALEONTOLOGY OF ILLINOIS. 



amalgamated. That there is no real fusion of the walls in 

 these cases is shown by abundant and conclusive evidence, fur- 

 nished, however, only by exceptionally preserved material. Or- 

 dinarily the visceral cavity is surrounded by concentrically 

 laminated sclerenchyma, while the central portion of the parti- 

 tion seems absolutely structureless. After a careful examination 

 of numerous sections, I find that this condition of parts is 

 mainly due to two causes, (1) imperfect preservation, (2) to 

 slight irregularities in the deposition of the internal laminae of 

 the sclerenchyma. The first cause needs no explanation, as 

 better specimens will show the median line in the wall which we 

 know to be double. The second occurs only in forms having 

 the partition rounded at the surface (e. g. Stenopora carbon- 

 aria, Worthen) and never in those in which they are carinate 

 (e. g. Anisotrypa symmetrica). When observed in longitudinal 

 sections, the partitions between the zooecia are seen to be com- 

 posed of a succession of superimposed conical layers, which are 

 deposited one upon the other as the growing margin of the 

 wall is carried upwards. When the centre of the partition is 

 carinate at the surface, then a more or less distinct straight 

 line passes through the apices of the conical laminae, but in 

 those forms that have the superficial termination of the par- 

 titions rounded, the separating line or primitive wall is often 

 made slightly zigzag by the alternate overlapping of the de- 

 posit on each side. This peculiarity sufficiently explains the 

 fact that while the divisional line may be quite distinct at one 

 point of a tangential section, it is scarcely or not at all per- 

 ceptible in other portions of the section. In considering the 

 wall structure of Bryozoa with numerous "mesopores," like 

 Callopora, Trematopora, Constellaria and Leioclema, we find 

 no difficulty in demonstrating the independence of the walls of 

 each zooecium, since, even when in contact with each other, a 

 divisional line is generally traceable. To show the duplex 

 character of the partitions between the mesopores, is a far 

 more difficult task, yet, that they are really double, will scarcely 

 admit of question. Like the thin-walled tubes of Monotrypa 

 and Diplotrypa, as well as the extremely thin partitions of the 

 tubes in the axial or immature region of nearly all the Palaeo- 

 zoic Bryozoa, the double condition of the mesopore walls is 

 conclusively shown by the fact that fractured surfaces almost 



