128 ANNALS NEW YORK ACADEMY OF SCIENCES 



rather than a new variety, the lower taxonoruic rank is probably more in ac- 

 cordance with the facts. 



The peculiar configuration of C. mesolobus is known to everyone. The 

 typical form is described as possessing fine, radiating stria?. Some well- 

 preserved specimens from Ohio show this feature very clearly. The lira? 

 are fine and moderately strong and they give rise to a large number of 

 minute spines, a feature not mentioned by Xorwood and Pratten, though 

 it is perhaps represented by one of their figures. Mr. Beede 6 also ap- 

 pears to record it when he describes the surface as "coarsely punctate." 

 He may, however, be referring to another and an altogether different 

 feature, to more numerous perforations, which occur between the lira? 

 instead of on them and which project as rows of spinules or pustules on 

 the inside of the shell. These are best shown on exfoliated specimens or 

 internal molds. The external feature to which I refer above has, also, 

 when the shell is worn, the appearance of punctae, but when better pre- 

 served, the punctae show projecting edges as of downward pointing spines, 

 very similar to the minute spines which are found on many orthoids. 



The variety which is the subject of this account has the characteristic 

 configuration of C. mesolobus, but the surface is entirely without radia- 

 ting sculpture, marked only on the best specimens by fine growth lines. 

 The absence of radial striation is not due to erosion or any circumstance 

 of fossilization, for it is a persistent feature shown by an extensive series 

 of specimens from many localities. Furthermore, the radial markings 

 could hardly have been lost, when the more delicate growth lines had 

 been retained. 



In characteristic specimens, this difference is so strongly marked that 

 one would be led to refer the two forms to altogether different groups, 

 and as already noted, I was at first disposed to regard them as distinct 

 species. When large series of specimens from different horizons are ex- 

 amined, how T ever, individitals more or less intermediate in character are 

 found. That is, associated with the smooth variety are occasional shells 

 which show faint yet unmistakable traces of radial sculpture. Such 

 specimens must be carefully examined, however, to determine whether 

 this character is not adventitious, for under exfoliation the rows of in- 

 ternal spinules which have oblique pores connecting with them form 

 little grooves and produce fine, more or less regular radiating markings. 



The shells belonging to the smooth variety rarely show traces of the 

 spines found on the other. Except for occasional incremental lines the 

 surface usually appears to be quite featureless. 



6 University Geo]. Surv. of Kansas, Rept., vol. VI, p. 71. 1900. 



