BRYOZOA. 555 



SEMICOSCIXIUM Prout, 1859. 



(Trans. St. Louis Acad. Sci. vol. 1, p. 443.) 

 (For generic diagnosis see page 395.) 



On account of certain very unfortunate errors in Prout's 

 original diagnosis of this genus, the name has not become cur- 

 rent. His type specimen was, however, almost beyond question, 

 a fragment of a large and easily recognized species which is 

 common at the Falls of the Ohio. He mistook the obverse for 

 the reverse side, and described the thin membrane which is often 

 drawn over the summits of the high carinae, as a longitudinally 

 lined sole. The spaces between the branches and this membrane 

 is filled with vesicular tissue. This he noticed and described, 

 together with the true zooecia, which he called "tortuous tubes," 

 as a part of the "sole." The cells, he supposed, originated in 

 the tortuous tubes and opened into the fenestrules. In short, 

 his conception of S. rhomboideum was altogether incorrect. 



The genus as now defined, departs from Fenestella in having 

 the keel very much higher and expanded at the summit, and 

 the branches zigzag on the reverse. The zoarium is always 

 infundibuliform. with the inner side non-celluliferous. 



Species of this genus can be recognized in the Niagara group, 

 but it is not until we reach the Upper Helderberg rocks that 

 they assume their most marked peculiarities and become abund- 

 ant. Here we find nineteen species, the majority of which have 

 been described by Prof. Hall under Fenestella. Seven more 

 species are described by that author from the Hamilton group. 

 Above this horizon the genus is not known. 



SEMICOSCTVIUM PLAXODORSATUM Ulrich. 



PI. XLIV, figs. 3-36. 



Zoarium infundibuliform above the abruptly contracted base, 

 with the free margins nearly flat, and somewhat undulated. 

 Outer surface celluliferous, strongly ridged. Branches rather 

 slender, sixteen or seventeen in 1 cm., 0.3 mm. wide just before 

 a bifurcation, increasing gradually to 0.5 or 0.0 mm. Dissepi- 

 ments very short, depressed, wider than the branches. Fene- 



