BRYOZOA. 207 
Eridotrypa exigua.] 
Verment and New York being mostly 1.0 mm. or a little more in diameter. Consid- 
ering the small size of the branches, the zocecia are large. Their apertures are 
oblique, but not excessively so, subequal, and arranged in both longitudinal and 
diagonal series, the former with eight in 3 mm., predominating in the small speci- 
mens, and the latter in the larger. At the lower end of each aperture the wall is 
usually raised into a spine-like prominence. In vertical sections the greater part of 
the branch is seen to consist of comparatively large and nearly or quite vertical tubes, 
intersected here and there by a diaphragm. The peripheral region is exceedingly 
short and abrupt. 
The small size of the branches might be regarded as indicating Bythopora rather 
than Eridotrypa, but the comparatively large zocecia and the internal characters, so 
far as known, point to the last genus with considerable certainty. Considered spe- 
cifically, its branches are so much smaller than those of even the var. minor of the 
preceding species that there is really no likelihood of confusion here. Both Bytho- 
pora herricki and B, alcicornis have smaller zocecial apertures and impressed, instead 
of ridge-shaped, interspaces. 
Formation and locality.—A large number of fragments were picked from washings of upper Galena 
shales collected from a locality near Cannon Falls, Minnesota. The eastern form of the species occurs in 
the Trenton limestone at Trenton Falls, New York, and Chimney Point, Vermont. 
Mus. Rey. No. 8129. 
Family HETEROTRYPID, Ulrich. 
Genus HETEROTRYPA, Nicholson. 
Heterotrypa (part.) NICHOLSON, 1879, Pal. Tab. Corals, p. 291; NICHOLSON, 1881, Genus Monticuli- 
pora, pp. 101 and 103. 
Heterotrypa, ULRICH, 1882, Jour. Cin. Soc. Nat. Hist.. vol. v, p. 155; ULRICH, 1882, idem, vol. vi, p. 83; 
Foorpb, 1883, Contri. Micro-Pal. Cambro-Sil. Rocks, Can., p. 20; 
ULRIcH, 1890, Geol. Surv. Ill., vol. viii, pp. 371 and 413. 
Zoaria frondescent, subramose, or incrusting. Zocecial tubes prismatic, some- 
times subcylindrical. Apertures angular, subcircular, or slightly petaloid. Walls 
moderately thin. Mesopores varying in number, sometimes abundant, with illy- 
defined walls. Acanthopores small, usually numerous. Diaphragms well developed, 
generally horizontal; occasionally a few may be concave or recurved. 
Type: 4H. frondosa VOrbigny, sp., (H. mammulata Nicholson, not d’Orbigny). 
This genus is not yet known to occur in the rocks of Minnesota, but the two 
species following are to be looked for in the southern part of the state, being rather 
common fossils in the Hudson River rocks of Illinois and Wisconsin, 
For remarks on this genus see under Dekayella, 
