1916] The Ottawa Naturalist. 87 



The pores penetrating the outer continuous sheet of the 

 mesostereom are directed perpendicularly toward the suture lines 

 between the plates, but incline more or less obliquely downward. They 

 apparently widen in a direction parallel to the inter-lamellar spaces in 

 passing through the outer sheet of the mesostereom, since, in strongly 

 weathered specimens showing the inter-lamellar spaces (Plate III), 

 the latter frequently appear interrupted by transverse partitions a short 

 distance below the outer continuous sheet of the mesostereom. At the 

 center of each thecal plate there is a space, at least a millimeter wide, 

 within which no trace of the vertical lamellae appears. 



14. Sections across the anal pyramid and the transverse apical 

 food-groove. A cross-section of the anal pyramid of Comarocystites 

 shows that the lower margin of the pyramid plates fits into a groove 

 extending along the lower part of the proximal margin of the bordering 

 thecal plates. The upper part of this proximal margin rises sufficiently 

 to admit of the presence of some substance for opening the anal 

 passage on the relaxation of the muscles holding the anal plates shut 

 from within the thecal cavity. 



The mouth, or opening into the interior of the thecal cavity, is 

 scarcely a millimeter in diameter, and is located at the posterior end 

 of the suture between the two anterior peristomial plates (a, a, in the 

 text diagrams). In form this opening varies from nearly circular to 

 more or less elliptical, with the longer diameter parallel to the direction 

 of the transverse apical food-groove. From this mouth the lateral 

 primary rays of the food-groove system diverge in opposite directions 

 in such a manner as to produce a slightly curved transverse continuous 

 groove across the apical end of the theca, with the convex side of the 

 groove directed toward the front. This transverse food-groove, be- 

 tween the bases of the arm pairs, is frequently exposed, but the central 

 mouth opening is rarely seen. Cross-sections perpendicular to the 

 length of the transverse apical food-groove in one specimen indicate 

 that the lower part of the posterior peristomial plates, projects slightly 

 beneath the adjoining part of the anterior peristomial plates, especially 

 toward the lateral extremities of this food-groove. To what extent this 

 feature is present in other specimens is unknown. 



15. The arms of Comarocystites punctatus. Each pair of arms 

 is supported by a single nodular stereom protuberance, but each 

 protuberance is supplied with two more or less divergent facets (see 

 facet 1, in fig. IB on plate II.) for the attachment of the arms. Each 

 end of the transverse apical food-groove, on coming in contact with 

 the adjoining protuberance, bifurcates, each branch of the food- 

 groove, together with its covering-plates, extending to one of the arm 

 bases, and then rising along the adoral side of the first brachial. 



Arms are known only in the case of two specimens, one found 

 and figured by E. Billings, the other found and figured by Sir James 



