74 The Ottawa Naturalist [Cctober 



plates extends sufficiently beneath the base of the protuberance 

 to suggest the origin of the latter as an accessory stereom de- 

 posit upon the surface of the theca, necessitated by the demands 

 for support made by the growing arms. 



The degree of compression of the undistorted theca is 

 moderate, the horizontal diameter from front to rear equalling 

 about .80 to .84 of the lateral diameter. Specimens preserved 

 in soft clay frequently present a much greater degree of com- 

 pression, due to distortion after death. The length of the theca 

 equals about ten-sevenths of the greatest transverse diameter. 



Text figure No. 1. Diagram of the thecal plates of the specimen represented by 

 figure 1 on plate II. The plates on the right of the vertical sinuous line on the 

 right side of the figure duplicate those at the left margin of the diagram. The 

 anterior peristomial plates are lettered a, a; the right and left posterior peristomial 

 plates are lettered rp and Ip respectively. The relative position of the different 

 arm facets is indicated by the numbers '2, 1 , 5 and 4, explained in the text. The 

 dotted line indicates diagrammatically the transverse apical food-groove which 

 forks at each end, each branch leading to the base of one of the arms, the latter 

 being arranged in pairs. The anal pyramid is indicated at A. The linear 

 hydropore extends from the middle of plate rp, diagonally downward and toward 

 the right, as far as the middle of the adjoining plate. 



Viewed from a direction at right angles to the plane of 

 symmetry passing vertically through the theca, and parallel 

 to the transverse apical food-groove, the sides of the theca differ 

 slightly in outline. On the anal side the outline is more angu- 

 larly convex, the maximum convexity being near micllength. 

 On the opposite side the maximum convexity tends to be dis- 

 tinctly less curved. This difference in outline evidently is due 

 to the location of the anus which has been dragged sufficiently 



