76 The Ottawa Naturalist [October 



between^the ends of the transverse food-groove. It is evident 

 that if an anterior ray ever was present in any of the ancestral 

 forms leading to Comarocystites, this ray may have rested on 

 the suture between the two anterior plates (between plates a, 

 a, of the text diagrams) here under discussion. The outline 

 of the right anterior peristomial plate is more or less obliquely 

 hexagonal, while that of the left anterior peristomial plate is 

 pentagonal. 



The posterior side of the transverse apical food-groove also 

 is bordered by two peristomial plates (Plate II, fig. IB; also 

 thecal plates lp and rp in text diagrams), of which the right is 

 so much larger that it forms about two-thirds of this posterior 

 border. The general outline of this plate is hexagonal, but the 

 apex of the angle on the left side is broadly truncated by a 

 concave curvature, as though three plates were in contact 

 with the left margin of this plate: a large, more or less hexa- 

 gonal plate along its lower left margin, and two more or less 

 quadrangular plates in contact respectively with the middle and 

 upper parts of this left margin. The line of contact between 

 these two quadrangular plates is not defined distinctly in any 

 of the specimens examined, but the upper one of these plates 

 borders on the left third of the transverse apical food-groove, 

 and may be described as the left peristomial plate. 



8. The location of the hydropore. The orientation of the 

 cystids is determined, not by the location of the mouth and 

 anus but by the vertical plane passing through the mouth and 

 hydropore. The hydropore is regarded as occupying a position 

 directly posterior to the mouth. In Comarocystites the only 

 surface structure suggestive of an entrance to a hydropore is 

 a narrow, sinuous, almost linear ridge, extending from the 

 middle of the right posterior peristomial plate (Plate II, fig. IB; 

 also thecal plate rp in text diagrams), across the suture on its 

 lower right-hand margin, to the middle of the adjoining plate. 

 The upper margin of the latter plate is in contact with the pos- 

 terior margin of that nodular stereom protuberance which sup- 

 ports the right pair of arms. Along the top of the narrow, 

 linear ridge there is a very narrow, faint groove, suggesting the 

 presence of a narrow slit-like opening. Whatever the homo- 

 logy of this ridge, it evidently locates the posterior side of the 

 theca. In several specimens there is a minute pit just beyond 

 the upper left-hand termination of this hydropore ridge; how- 

 ever, since it was not observed in the majority of specimens, it 

 cannot be determined definitely as a gonopore. 



Nothing suggesting a hydropore is known at present in 

 Amygdalocystites. In Canadocystis emmonsi, however, ,G. H. 

 Hudson (N.Y. State Museum 'Bulletin 80, 1905, pp. 273, 274) 



