BRYOZOA. 455 



Its very much thinner zoarium, the rhombic cell apertures, 

 their regular arrangement and the almost total lack of dia- 

 phragms discriminate this form from P. compressa. 



Position and locality: Hamilton group; Buffalo, Iowa and 

 Rock Island, 111. 



LEPTOTRYPA Ulrich, 1883. 



Am. Pal. Bry. Jour. Gin. Soc. Nat. Hist. vol. VI, p. 158. 

 (For generic diagnosis see page 377.) 



My original diagnosis of this genus included only thin para- 

 sitic zoaria, but later investigation has shown that these could 

 not be separated from several species that differed only in the 

 thickness and ultimate form of the colony. The zocecial features 

 are identical in the massive and parasitic forms, nor is there 

 any difference in the general plan of construction. 



Tho massive and discoid species present great resemblance to 

 MonotrypaNich., but good tangential sections serve to distinguish 

 the genera at once, acanthopores being a conspicuous feature in 

 sections of Leptotrypa, while such structures are wanting in 

 Monotrypa. In all about seventeen species of Leptotrypa are 

 known to me from American Palaeozoic rocks; of these one 

 (Chsetetes quadrnngularis Nich.) or perhaps two occur in the 

 Hamilton group, two in the Niagara, and the rest in the Cin- 

 cinnati and Trenton groups. The four species here described are 

 fair representatives of this genus. 



LEPTOTRYPA HEXAGONALIS Ulrich. 



PL XXXVI, Fig. 6-6a. 



Zoarium forming parasitic expansions less than one mm. in 

 thickness, spread upon Orthoceras or Hyalithus. Surface smooth. 

 Clusters of cell apertures of almost twice the usual size are 

 arranged in diagonally intersecting rows; these clusters are about 

 three mm. apart, measuring from center to center. Zocecia 

 regularly hexagonal in shape, sometimes a little elongated, 

 seven measuring longitudinally, almost nine, diagonally, in two 

 mm., diameter of the smaller 0.2, of the larger 0.35 mm. Acan- 

 thopores prominent on the surface when well preserved. 



