Lower Helderberg Bryozoa and Corals. 97 



twice or more than twice their width. Pores distinct, rounded, their 

 margins very slightly elevated, scattered on the wider but forming 

 rows in the narrower parts of the branch ; two, three or four in the 

 breadth of the brancli. On the dissepiment, near its junction with 

 the branch, a single pore is often visible, but the dissepiments have 

 not a poriferous character. 



This beautiful species is the only true Polypora yet known in the 

 Silurian strata of New York. It is easily distinguished from the 

 associated Fenestella by the irregular mode in which the branches 

 multiply, and by the greater number of pores, without dividing 

 ridges. The poriferous side, as seen in the rock, has somewhat the 

 appearance of Retepora asperato-striata of the Niagara limestone, 

 except that it has a finer and more delicate structure, fewer pores, 

 and the dissepiments are destitute of cells except at the extremity. 



Formation cuid locality. — On weathered slabs of the Lower Hel- 

 derberg limestone, at Schoharie and near Clarksville, New York. 



Polypora elegans (?) n. sp. 



Bryozonm growing in small, irregular and duplicating tan-shaped 

 overlapping fronds, attached by the base to foreign substances. 

 Branchlets slender, rounded, frequently and irregularly bifurcating 

 and rapidly diverging. Dissepiments strong, two-thirds as thick as 

 the branches and placed at irregular distances. Fenestrules a little 

 longer than wide. Pores small, round or ovate, comparatively dis- 

 tant, with distinctly elevated margins ; arranged in two series only, 

 and alternating, one on each side of the branch, leaving a smooth or 

 striated space between ; the distance between the two ranges equal 

 or nearly equal to the diameter of the pore with its margin. Surface 

 between the pores, and also the dissepiments, striated with tortuous 

 striae. Opposite side not known. 



This species is remarkable as having only two ranges of pores, 

 although with all the other essential features of the genus Polypora. 



Formation and locality. — In shaly limestones of the Lower Hel- 

 derberg group, near Clarksville, Albany county. New York. 



Genus HEMITRYPA Phillips. 



The genus HEMriRTPA of Phillips was established for a group of 

 Fenestelloid bryozoans, which have unusually high ridges dividing 

 the ranges of pores on the exterior of the frond, and the crests of the 

 ridges connected by a series of dissepiments which are not unfre- 

 quently so much thickened laterally as to leave only small round 



