American Palaeozoic Bryozoa. 



125 



walls of the tubes in the " immature" or axial region are certainly en- 

 tire and without perforations of an}' kind. The " connecting foramina" 

 are preserved in only a portion of the specimen alluded to, for in sec- 

 tions taken from other portions of the same specimen, thej T are either 

 vfcry obscurely preserved, or no traces of them whatever can be detected. 

 From the fact that so man} r species, more or less nearly allied to M. 

 obliqua, have been most carefully examined, and no traces of connect- 

 ing foramina found, their discovery in a single example of M. obliqua 

 must be looked upon in the light of a fortunate accident. Why they 

 have not been detected in many other species of the Monticuliporidce, 

 I can not certainly decide, but I will suggest that the foramina were 

 in use, or I should rather say open, only in the outermost region of the 

 zoarium, i. e., between'the last diaphragm and the cell-aperture. As 

 growth proceeded, and a new layer of cells was developed, the walls of 

 the preceding layer of cells were thickened by la} T ers of sclerenchyma, 

 which of course closed the minute connecting foramina. The foramina 

 would consequently be shown only in sections of such specimens, or 

 portions of same, in which the secondary layers of sclerenchyma were 

 of a lighter or darker color than that of the layers forming the original 

 boundary of the cell. 



(c.) Immature or Axial, and Mature or Cortical Portions of the 

 Tubes. — In all cases, whatever may be the structure of the walls of the 

 tubes in their final and most developed condition, they commence 

 with thin and apparently indivisible walls. This I have called the 

 M immature" portion of the tubes, and in the ramose and frondescent 

 forms, occupies the axial and deeper regions of the zoarium, and 

 almost invariably terminates at, or very near, the point at which the 

 tubes bend more or less abruptly outwards in their course to the sur- 

 face. In the " immature" region of the zoarium, the diaphragms are 

 often entirely wanting, and always more remote than in the " mature" 

 or cortical region. C3 r stoid diaphragms and spinifo.rm tubuli are 

 never developed in this region, nor are the true interstitial tubes, all 

 three of these structures first making their appearance in the cortical, 

 or what I would call the "mature" region. The peripheral portion 

 of the zoarium, in the great majority of the forms under consideration, 

 differs more or less conspicuously from the "immature" or axial 

 region just described. The tubes bend outwards, the walls become 

 more or less extensively thickened, and if at all present, the cystoid 

 diaphragms, the interstitial tubes, the spiniform tubuli, and the con- 

 necting foramina, arc developed ; besides, the diaphragms become more 



