570 PROCEEDINGS OF THE NATIONAL MUSEUM. 



was is not known, but, as suggested by Ulrich, they may have sup- 

 ported appendages similar to the avioularia or vibracula of recent 

 bryozoa. They certainly are not, as considered by Waagen and Went- 

 zel,«mere thickenings preceding gemmation. Nor do they give rise 

 to mesopores, which in turn develop into true zooecia as schematically 

 illustrated by these authors. In actual sections such a development 

 is not known, and among the thousands of thin sections of trepostoma- 

 tous bryozoa examined by the writer, not one has been seen that in 

 anywise countenanced their interpretation. The zooecia, mesopores, 

 and acanthopores of these fossil forms are as distinct and independent 

 structures as are the polypides, avicularia, and vibracula of recent 

 chilostomatous bryozoa, whose wholly distinct morphological develop- 

 ment has been demonstrated by students of living species. 



COMMUNICATION PORES AND INTERMURAL STRUCTURE. 



Under certain conditions tangential sections indicate that the zooecial 

 walls and the intermural space are seemingly pierced by communica- 

 tion pores or connecting foramina. These were first recorded by 

 Ulrich in the descriptions of H. curvata and H. ohliqua. Dr. Rom- 

 inger, in a critical paper,* subsequently denied the presence of these 

 pores. I have figured tangential sections of H. wm'theni and TI. nodu- 

 losa (Plate XXIV, fig. 12; Plate XXV, fig. 15) which show that such 

 structures do exist in Ilomotrypa and are found in other species than 

 those mentioned l)y Ulrich. Indeed, they have been found in so many 

 species of this genus that their presence may be considered a generic 

 feature. These pores are best observed when the section is rather 

 thick. As the section is thinned the pores become less distinct, and 

 when very thin disappear altogether. This is because the denser 

 tissue of the zooecial walls in thick sections brings out into relief thei 

 clearer substance of the connecting foramina, but as the section is' 

 thinned the substance of the walls becomes more and more translucent, 

 until, finally, both wall and pore are alike in clearness and the outline 

 of the latter is lost. 



Thin sections show that each zocecium has its own bounding wall dis- 

 tinct from adjacent zooecia, the space between, which is here termed 

 the intermural space, being occupied hj the acanthopores and a dotted 

 or granular layer, which in sections has a structure very similar to 

 that exhibited by the parenchymal chord of more recent bryozoa (see 

 Plate XX, fig. 2). The width of this intermural space varies with th( 

 species and with the age of the zooecia. Sometimes, even in the f ullj 

 matured condition, it is represented by merely a fine granular lin( 

 (Plate XX, fig. 1), but in some species (see H. aicstini, Plate XXIV 



« Paleontologia Indica, 13th Ser., XIII, 1886, pp. 861, 871. 



i> Studies on Monticulipora, American Geologist, VI, 1890, p. 118, 



