BRYOZOA. 267 



Erldotrypa exigua.] 



Vermont 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 zooecia 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 zooscial 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. Reg. No. 8129. 



Family HETEROTRYPID.E, 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; 



FOOBD, 1883, Contri. Micro-Pal. Cambro-Sil. Rocks, Can., p. 20; 



ULRICH, 1890, Geol. Surv. 111., vol. viii. pp. 371 and 413. 



Zoaria frondescent, subramose, or incrusting. Zooecial 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 : H. frondosa d'Orbigny, 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, 



