PALAEONTOLOGY OF IOWA. 479 



Striatopora rugosa ( n. s.). 



PLATE I. FIG. 6. 



COKAL ramose, frequently bifurcating ; branches very gra- 

 dually tapering ; cells somewhat distant, arranged in alter- 

 nating series, subcylindrical below and gradually expanding 

 towards the aperture, with the walls much elevated on the 

 basal and lateral margins. 



This species is much more robust than the S.flexuosa of the Niagara group, with 

 stronger and more rigid branches : the cell apertures are smaller, less angular, and 

 more distantly arranged. 



The specimens examined are all more or less worn, and the surface characters not 

 well preserved. Some of the cells show the interior striation which characterizes the 

 genus. There is a similar species in the shales of the Hamilton group in New-York. 

 Fig. 6. A fragment of this fossil : natural size. 



Geological formation and,. locality. In shaly limestones of the age of 

 the Hamilton group of New- York : New-Buffalo, Iowa. 



6RDJOIDU! OF THE HAMILTON GROUP. 



GENUS ACTmOCRIMJS (MILLER). 



[ SUBGENUS ] MEGISTOCRINUS (OwEN & SHUMABD). 



BODY depressed, subspheroidal or broadly urnshaped : 

 basal plates three, usually anchylosed, and scarcely extend- 

 ing beyond the edge of the column ; second series of plates 

 (often appearing as the basal series) six, five radials and 

 one anal : radial plates three in five series, interradial 

 plates six to ten or more in five series ; anal plates seven to 

 thirty or more ; each upper radial supporting two and some- 

 times three supraradial plates : from each upper sloping 

 side, supraradials, often in double series above the first plate ; 

 brachial plates numerous ; arms composed of numerous thin 

 joints arranged in double series ; subaxillary plates none to 

 six or nine. 



