American Palaeozoic Bryozoa. 



145 



the axial region of the branches, and then curve outward more or less 

 abruptly to reach the surface. In both, therefore, there are established 

 two distinct regions, an axial ( ,k immature"), and a peripheral (" ma- 

 ture") region. In both, these two regions are very different in their 

 internal structure, the tubes in the axial region of their course being 

 thin-walled and polygonal, while in the peripheral region their walls 

 are thickened, and they often become rounded in form. In both, more- 

 over, the interstitial tubes that may be present, are developed in the 

 peripheral region only, and they do not extend into the axial region 

 at all. 



(2). As regards the dimorphism of the zoarium of such types of the 

 Monticuliporidoe as Batostomella, Leioclema and Callopora (?) Cin- 

 cinnati ens is , we find that they consist of two distinct sets of tubes, 

 which differ from each other (1), more or less in size; (2), in one set 

 having more numerous diaphragms than the other; and (3), in their 

 time of development, the smaller or interstitial tubes being developed 

 only in the peripheral region. A fourth distinction is presented in Leio- 

 clema; the cavities of the true tubes in that genus being surrounded 

 by a series of spiniform tubuli. In Heteropora the zoarium similarly 

 consists of a series of large tubes surrounded by smaller interstitial 

 tubes, and with the exception of one feature, the same differences 

 between the two sets of tubes are noted. The characters excepted, is 

 that in Heteropora, so far as 1 have been able to determine from 

 actual examination, diaphragms are usually absent in the peripheral 

 region, and consequently in the interstitial tubes they are entirely 

 wanting. That this difference should be considered of importance in 

 the determination of the real question at issue (i. e., the zoological 

 position of the Monticuliporidai and allied types), I can not admit, 

 since transverse partitions occur in organisms of such exceedingly 

 diverse affinities, that we can not attach much value to the fact that 

 they appear to be absent in a portion of the zoarium of Heteropora. 

 Besides, I believe that diaphragms were developed even in the peripheral 

 region of the tubes of that genus, and I attribute their absence to the 

 supposition that the opercular plates which closed the cell-apertures^ 

 and which subsequently formed the base of a new la3 r er of cells, were 

 of such a nature that they were incapable of preservation during 

 fossilization. This supposition is made a probability by the fact that 

 in the recent species, JET. neozelanica, Busk, the cell-mouths are closed 

 by a thin chitinous covering, which in the fossil state would scarcely 

 have been preserved. 



