„ , „ BEYOZOA. 229 



HomotTypella nstabilis.] 



The latter continue into the Lower Helderberg and may really be the stock that 

 eventually produced Batostomella annulata {Trematella annulata Hall) and B. verspin- 

 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. 



HOMOTRTPELLA INSTABILIS Ulrich. 

 PLATE XVIIl, FIGS. 9-20. 



Homotrypella instahilis 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 maculae 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 zooecial apertures are irregular both in form and arrangement, with thin walls, 

 partly separated by mesopores numbering one or two to each zooecium. In these 

 specimens the acanthopores are small yet prominent and sharp, and number from 

 one to three to each zooecium. 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 suflBces 

 to obscure the mouths of the mesopores, i.o that they are readily overlooked. Twelve 

 or thirteen of the zooecia 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 zooecial 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 zooecial cavity open, is to be 

 seen in each of the zooecia. 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. 



