362 CUMINGS AND GALLOWAY MORPHOLOGY OF TREPOSTOMATA 



cingulum (9), because in tangential sections (figures 23 to 36) they give 

 the appearance of a well defined ring or zone of deposits adjacent to the 

 zooecial cavity. The cingulum is shown in longitudinal section in figures 

 43, 44, 46, 47, 56, and 58. In figures 45 and 48 there are also massive 

 secondary deposits, which do not, however, form so sharply defined a layer 

 as is shown in the other sections. Tliese secondary deposits are inti- 

 iiuiiclv rchitcd lo |lic (li;ii>liragnis. as llie figures clearly show. The dia- 

 phragms and the ciiignhnti nrc coiil Iiiikmis (figures 44, 46, 47, and 45). 

 Ft appears, fliprefore, that the secretion of ;i diapfiragm is coincident with 

 the formation of a deposit over tlie entire inner surface of the zooecium. 

 The extremes to which this secondai-y tliickening may go are illustrated 

 in figures 1, 9, 11, 17, 45, and 47. An unusually extreme example is 

 Diplotrypa hicornis, figured by Bassler in his volume on the Baltic 

 Bryozoa (4). 



The fact has already been pointed out that the cystiphragms may be 

 very closely related to this secondary thickening of the walls. This is 

 beautifully illustrated in figure 17. Thick-walled species with cysti- 

 phragms (for example, HomotrypeUn) afford numerous illustrations of 

 the same thing. In figure 17 the boundary between the primitive wall 

 and the secondary deposit is clearly shown by a distinct line, and the 

 thickened cystiphragms have the same definite relation to the cingulum 

 as do the diaphragms in the cases already cited. 



The thickening of the interzooocial walls, due to the development of 

 the cingulum, is often very great — far greater than would merely com- 

 pensate for the increasing separation of the zocecia, as they extend radially 

 outward from the axial region. There is an actual reduction of the size 

 of the zooecial chamber; indeed, in some cases, an extreme reduction 

 (figures 1 and 17). We believe that this extreme development of sec- 

 ondary deposits is a senile feature, analogous to the great thickening of 

 brachiopod shells and the shells of the Mollusca in old age. In recent 

 Bryozoa the zooecia of the older portions of zoaria often become almost 

 or quite filled up with stony deposits, and it seems that the ectosarc may 

 continue to secrete such deposits after the polypide has wholly disap- 

 peared from the zooecium. 



We can not dismiss this subject without calling attention to the striking 

 resemblance between the wall structure of the Trepostomata and of the 

 Brachiopoda. In such sections as are shown in figures 45 and 49 it 

 amounts almost to identity. In figure 54 also the fascicles of wall laminae 

 look almost precisely like the similar fascicles of lamina? so often seen 

 in sections of brachiopod shells. 



