1 1 



7* 





THE OTTAWA NATURALIST 



Vol. XXX. NOVEMBER, 1916. No. 8. 



COMAROCYSTITES AND CARYOCRINITES. 



Cysttds with pinnultferous free arms. 



By A. F. Foerste, Dayton, Ohio. 



(Continued from page 79.) 



13. The structure of the thecal plates. The exterior surface of 

 the thecal plates of Comarocystites punctatus is deeply concave. The 

 interior surface, however, appears more or less stellately convex. The 

 convex appearance is due, in part, to the slopes of the suture planes, 

 converging toward the center of the theca, and, in part, to the thinning 

 of the plates toward the angles of their polygonal outlines. The 

 stellate character is due to grooves separating the different sets of 

 mesostereom plates, described later in this paragraph. These grooves 

 narrow toward the angles, thus increasing the stellate appearance. 



In cross-sections which are vertical to the surface of the thecal 

 plates and perpendicular to the middle parts of the sutures between the 

 plates, the inner surface of the plates presents an almost straight out- 

 line between the center of one plate and the center of the next, or there 

 is a moderate outward bending of this outline at the suture. However, 

 toward the angles where three plates meet, the inner surface of the 

 plates curves so strongly outward as to produce the appearance of 

 deep triangular pits at these points of junction. Owing to the deep 

 concavity of the exterior surface of the plates, the thickness of these 

 plates varies from five-tenths to six-tenths of a millimeter at the 

 center to nearly two millimeters along the middle of the suture lines. 

 Toward the angles, however, where three plates meet, and where the 

 inner surface of the plates curves strongly outward, so as to approach 

 the outer surface, the thickness of the plates frequently is reduced to 

 about a quarter of a millimeter. Viewed from the interior of the theca, 

 with the plates still connected, the deep triangular pits or depressions 

 between the ends of the stellate rays characterizing the individual 

 plates, form the most striking features. 



