106 The Ottawa Naturalist [December 



the transverse apical food-groove, on the adoral side of the protuber- 

 ance, and traces of the facets for the attachment of the arms are 

 preserved. The presence of vertical plates belonging to the meso- 

 stereom is seen along the strongly weathered sutures between the plates. 

 Several of the plates present very clear evidence of the arrangement of 

 the pores, through the continuous exterior surface of the mesostereom, 

 in pairs, and directly beneath the epistereom these pores evidently 

 are elongated in a direction parallel to the narrow spaces between the 

 mesostereom plates beneath- 



23. The structure of the thecal plates. A fuller knowledge of 

 the plate structure of Comarocystites shumardi is presented by the 

 specimens belonging to the Walker Mjseum, at Chicago University, 

 and by the specimens belonging to the Illinois State Museum of 

 Natural History (Plate IV, figure 3). The structure evidently is 

 identical with that of Comarocystites punctatus. There is the same 

 grouping of pores traversing the mesostereom. The thin epistereom is 

 non-porous, but when weathered away the outer terminations of the 

 pores traversing the mesostereom are seen to be arranged in more or 

 less alternating pairs. Directly beneath the epistereom, each of these 

 pores is connected with a semi-lunate pore- parallel to the outer surface 

 of the plate, the concave sides of each of the semi-lunate pores, be- 

 longing to the same pair, facing each other. As in Comarocystites 

 punctatus, some specimens show no indication of the presence of these 

 pairs of semi-lunate pores on their exterior surfaces; in others, their 

 presence is indicated by low, short, semi-lunate ridges. The meso- 

 stereom consists chiefly of more or less vertical plates, from 6 to 9 in a 

 width of 3 millimeters, intercepted by much narrower spaces apparently 

 connected directly with the interior of the theca without the interven- 

 tion of a hypostereom. Directly beneath the epistereom, however, the 

 mesostereom forms a continuous sheet penetrated only by the pores 

 connecting the narrow spaces between the vertical mesostereom plates 

 with the semi-lunate pores immediately beneath the epistereom. The 

 thecal plates appear to have grown from the margin outward, so that 

 the pores originating at the sutures later were located in the more 

 central parts of the plates. 



24. Horizon and Distribution of Comarocystites shumardi. 

 From the preceding statements it is evident that Comarocystites 

 shumardi is a typical representative of the genus Comarocystites. The 

 so-called variety obconicus is founded, it is believed, upon individual 

 characteristics, and the name should not be retained, even as the name 

 of a variety. 



Both Comarocystites shumardi and its so-called variety obconicus 

 were described from the Kimmswick, limestone- at Cape Girardeau, 

 Missouri. By Ulrich, this Kimmswick limestone is placed at the top 

 of the Black river group- beneath the Curdsville horizon at the base of 



