546 Keport of the State Geologist. 



Thamnotrypa, Hall. 

 " (Pal. K r., Vol. YI, p. 101, PI. 33, figs. 9, 10.) 

 Type, Thamnotrypa divaricata, Hall. 



(Plate 12, figs. 7, 8.) 



Zoarium consisting of a very narrow stipe, from which pro- 

 ceed, rectangularly, lateral branches ; celluliferous on each face ; 

 cell apertures oval ; usually disposed in two parallel longitudinal 

 rows, separated by a prominent ridge. Sometimes three rows 

 occur and occasionally four rows for a short distance on the 

 stipe. On the lateral branches there are never more than two 

 rows. 



Family Arthroclemidae. 



Zoariuna articulated, ramose ; consisting of numerous subcylin- 

 drical segments ; cells sub tubular, more or less oblique ; radially 

 arranged around a central axis, opening on all sides of the seg- 

 ments. The family includes the folio wing genera : Arthroolema, 

 Helopora and (?) Sceptropora. 



Arthroolema, Billings. 



(Pal. Foss., Yol. I, p. 54. 1862.) 



Type, Arthrodema pulchella, Billings. 



Zoarium jointed, composed of numerous 'subcylindrical seg- 

 ments ; celluliferous on all sides ; arranged in a pinnate manner ; 

 articulation both terminal and lateral. The segments *are of 

 three kinds, primary, secondary and tertiary. The primary 

 segments form the strong central stem, of which each seg- 

 ment has normally one or two sockets on opposite sides, for 

 articulation with the secondary segments, which usually articu- 

 late in like manner, terminally with each other and laterally with 

 still more slender tertiary segments ; cells sub tubular, apertures 

 ovate, oblique, the lower border more or less prominent ; disposed 

 in longitudinal rows separated by ridges. 



