BRYOZOA. 229 
Homotrypella nstabilis.] 
The latter continue into the Lower Helderberg and may really be the stock that 
eventually produced Batostomella annulata (Trematella annulata Hall) and B. perspin- 
ulata Hall, sp., of the Devonian, and B. spinulosa Ulrich, the Chester type of the 
genus. But I am satisfied that the gracilis group, indeed the whole genus Homotry- 
pella, did not survive into the Niagara. 
Homorrypenia instaBiiis Ulrich. 
PLATE XVIII, FIGS. 9-20. 
Homotrypella instabilis ULRICH, 1886. Fourteenth Ann. Rep. Geol. and Nat. Hist. Surv. Minn., p. 83. 
Zoarium ramose, growth rather irregular ; branches rounded, sometimes nodular 
or lobate, and varying in diameter from 3 to 8 mm.; surface generally without mon- 
ticules, and when these are present they are low and broad ; small maculz or clusters 
of mesopores are not infrequently present. Superficial characters of zocecia and 
mesopores variable. In some, and these are in most cases well-preserved examples, 
the zocecial apertures are irregular both in form and arrangement, with thin walls, 
partly separated by mesopores numbering one or two to each zocecium, ' In these 
specimens the acanthopores are small yet prominent and sharp, and number from 
one to three to each zocecium. The mesopores are always smaller than the zocecia, 
but vary occasionally in shape, size and arrangement. In many other examples 
both the zocecia and mesopores are smaller and their walls correspondingly thick, 
while the acanthopores are blunt and thicker. In most cases a little wearing suffices 
to obscure the mouths of the mesopores, so that they are readily overlooked. Twelve 
or thirteen of the zocecia occur in 3 mm. 
Internal characters: As may be seen by comparing figures 13 and 14 with 18 and 
20 (plate XVIII) tangential sections of this species present an unusual variety of 
appearances. In the majority of sections, providing they are not too deep, the walls 
of the cells are very thick, with not a sign of cystiphragms in the zocecial cavities. 
When a second or peripheral series of cystiphragms has been developed (see figs. 
17-20) a very different appearance is obtained. Now the walls are thinner, and a 
cystiphragm, leaving from one-third to one-half of the zoccial cavity open, is to be 
seen in each of the zocecia. In all cases the polygonal lines of contact between the 
two sets of cells is sharply defined, and the walls of both approximately of equal thick- 
ness. The acanthopores are conspicuous features of these sections, but their rela- 
tive abundance varies somewhat in different examples. In the axial region of verti- 
cal sections the walls of the tubes are very thin and finely wavy, and the diaphragms 
straight and remote, or wanting entirely. As the tubes enter the peripheral region 
the number of diaphragms is greatly increased, the walls thickened, and cystiphragms, 
