Lower Helderberq Brtozoa and Corals. 105 



sions : surface of branches smooth, having a worn appearance. Cell 

 apertures small, round or slightly elongate, distant from each other 

 once and a half to twice their own diameter; with a narrow 

 depressed space surrounding the aperture ; intercelhilar spaces 

 smooth or but slightly channeled. On smooth silicified specimens 

 the dermatic film, covering the intercellular spaces, is often thin and 

 translucent, with the appearance of opercula caused by the refraction 

 of light from the small cavities beneath. Inner surface of the 

 branch-tubes transversely wrinkled and obscurely striated longitudi- 

 nally by the recumbent portion of the cell tubes. 



Formation and locality. — On weathered surfaces of the shaly 

 limestones of the Lower Helderberg group, near Clarksville, 

 Albany county, ISTew York. (Rather abundant.) 



Trematopora corticosa n. sp. 



Bryozoum growing in long, irregular, solid branches, with distant 

 ramifications, which are often more than an incli apart, and diverging 

 at an angle of about ninety degrees; diameter of branches one-eighth 

 of an inch or less. Cells numerous, elongate-ovate, sometimes elon- 

 gate-hexagonal, deeply impressed. Intercellular spaces strongly 

 elevated, forming roughened ridges between the cells, like the minia- 

 ture roughened bark of a tree. 



This species is easily recognized by the rough surface and distant, 

 widely-diverging branches. It is not very abundant. 



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

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



Trematopora densa n. sp. 



Bryozoum growing in solid, ramose branches, an eighth of an inch 

 or less in diameter; their surface closely covered by small polygonal 

 cells, with narrow, flattened interspaces. Cells rising from the center 

 of the branch and rapidly diverging to the exterior; cell apertures 

 various in size and irregular in position. Intercellular spaces solid 

 on the exterior, cellulose within ; the surface at the angles between 

 the adjacent cells elevated, rising into small node-like tubercles. 



In general aspect this species has the appearance of a small finely 

 marked Chretetes ; but where broken so as to show the hollow tubes 

 and cellulose intertubular space, its character as a Trematopora is 

 clearly marked. In size and general appearance it has some resem- 

 blance to T. corticosa, but is easily distinguished by the smaller and 

 less elongate cells, and less elevated intercellular substance. 



