Lower Helderberg Fauna. 289 



istic Silurian genus. Its presence here is important in its bear-' 

 ing upon the position of the Lower Helderberg Group in the geo- 

 logical scale, for similar forms have been found in the earliest 

 Devonian faunas of Germany. 



The stipe is cylindrical, sometimes tetragonally prismatic. It 

 is hollow, with thick walls, crossed at intervals by tabula, one 

 for each serration, which bound the zooidal habitations. On op- 

 posite sides, there are projecting ridges, or costee, the plane of 

 the latter being at right angles to the plane of the serrations. 

 Besides the costae, there are often fine longitudinal grooves or 

 striae. The teeth are rather distant ; the upper side is sometimes 

 at right angles to the stipe, at others acutely inclined to it. The 

 upper surface is somewhat flattened and is pierced by circular or 

 elliptical zooidal openings, situated well toward the stipe. The 

 under surface of the teeth is sometimes flattened, sometimes 

 rounded, the flattened examples at least being provided with a 

 median ridge. The specimens observed are all fragmentary, and 

 the nature of the sicula has not been ascertained. No indica- 

 tions of branching have been noticed, but the zoarium may be 

 bifurcated or even ramose. 



Horizon. — Shaly limestone, Indian Ladder, Helderberg 

 Mountain. 



STROMATOPOEOIDEA. 



With the exception of Actinostroma Bistigouchense, Spencer, I 

 have been unable to find notices of any American Stromato- 

 poroids from the Lower Helderberg Group. The species de- 

 scribed below constitute only a beginning in the investigation of 

 this class of Coelenterates, which is well represented in the life 

 of this period. But to make an exhaustive study of this portion 

 of the fauna would require a large collection, gathered with that 

 end especially in view. 



All the Stromatoporoids in this collection, with the exception 

 of GlatKrodiciyon Jewetti, appear to be formed on one type of 

 structure, and I have referred them, but with some hesitation, to 

 the genus Syringostroma, Nicholson. This genus (type S. densum) 

 was proposed by Nicholson* for some Stromatoporoids from 



* Nicholson, 1875. Geol. Surv. Ohio, Pal., vol. II, p. 251. 



37 



