1916] The Ottawa Naturalist 105 



In one specimen (Diagram No. 6) showing the transverse apical 

 food-groove very well, the posterior margin of this food-groove appears 

 occupied exclusively by the plate marked rp in the diagram. The 

 stereom protuberances, supporting the arm pairs, appear to rest upon 

 the margins of the adjacent thecal plates. These stereom protuber- 

 ances appear to be deposits made by the bases of the arms at the ends 

 of the transverse apical food-groove, and not to be a part of the thecal 

 plate system. The peristomial plates, on the contrary, are ordinary 

 thecal plates. Judging from the presence of small plates along the 

 margin of the stereom protuberances in some specimens, and their 

 absence in otkers, these small plates may be additions during the later 

 stages of growth of the individual. 



In Comarocystites punctatus the number of thecal plates in a 

 vertical series often numbers 9 or 10; in Comarocystites shumardi this 

 number usually is only 6 or 7. The theca grows in size chiefly by 

 growth at the margin of the individual thecal plates. It is quite 

 evident from the absence of small intercalated plates in some of the 

 specimens at least that the enlargement in growth does not depend 

 upon the introduction of intercalated plates within the general body 

 of the theca, although it is not impossible that additional plates, during 

 earlier stages of growth, may be added at the base. The evidence in 

 favor of such a suggestion is not very clear and consists chiefly in the 

 presence, at the base, of plates of small size inserted between those of 

 larger size. 



22. The so-called variety obconicus. Meek and Worthen prob- 

 ably were in error in attempting to distinguish a variety obconicus, as 

 distinct from Comarocystites shumardi. Close examination of the 

 type specimen (Figure 2a, on plate 1, Geol. Surv. Illinois, vol. Ill) 

 fails to show any distinguishing features excepting that presented by 

 the more attenuate base. As a matter of fact, however, there is no 

 evidence that this attenuate base is anything more than an individual 

 characteristic. The second specimen figured by Meek and Worthen 

 under the variety name obconicus (Figure 2b, on plate 1, of the Illinois 

 report cited above) does not differ in any respect from ordinary speci- 

 mens of Comarocystites shumardi, and certainly does not possess an 

 obconical base. The first specimen presents clear evidence of the 

 division of the mesostereom into vertical plates, shorter toward the 

 angles of the plates, and separated by very narrow interspaces. The 

 column has a width of 2.8 millimeters, and 17 columnals of about 

 equal size occur in a length of 5 millimeters. The surface of the 

 column is minutely granulate, as in Comarocystites punctatus. The 

 second specimen does not differ in any respect from small specimens 

 of Comarocystites shumardi. Only the left half of the theca is exposed 

 but this half includes all, from the base to the stereom protuberance 

 supporting the left pair of arms. Even the forking of the left end of 



