298 s. weller — Fauna of the ferx glex formation 



mesial sinus. Brachial valve concave, following rather closely the curva- 

 ture of the pedicle valve: in none of the Fern Glen examples have the 

 cardinal spines been observed. Surface of both valves marked by fine, 

 nearly regular, radiating costae, which increase by bifurcation on the 

 pedicle valve and by intercalation on the brachial valve ; the total number 

 of costae vary from 175 to 225, according to the size of the individuals, 

 but there are constantly about 6 costae in the space of 1 millimeter. 



The dimensions of two individuals, the larger of which is slightly flat- 

 tened, are: Length. 13 millimeters and 12 millimeters; width, 21 milli- 

 meters and IT. 5 millimeters: convexity, 3.5 millimeters and 4.5 milli- 

 meters. 



Remarks. — There has been some confusion in the interpretation of this 

 species as it occurs in the Kinderhook and Osage faunas of the Missis- 

 sippi valley and errors have sometimes been made in the identification 

 of the Kinderhook specimens. In its typical form the species occurs in 

 the Burlington limestone, sometimes in enormous numbers, these speci- 

 mens usually having a width of about 15 millimeters or a little more. 

 Where the species occurs more sparingly it often grows to a larger size, 

 sometimes attaining a width of 22 millimeters. In TTorthen's descrip- 

 tion of the species the number of costae are said to be 100 to 120 or more 

 in number, but this number is understated, as about 1T0 to 190 costae are 

 the usual number upon shells of average size, but upon larger individuals 

 they increase to 225 or more. The actual number of costae varies with 

 the size of the individual, but the size of the costae is a much more con- 

 stant feature, there being about six in the space of one millimeter, 

 whether the shell be large or small. In Meek and Worthen's description 

 of the species 12 to 14 costae are said to be present in .10 inch, which 

 would be nearly the number observed on the specimens studied by the 

 writer; 6 costae in one millimeter would be equivalent to 15 in .10 inch. 

 WinchelTs C. multicosta. with "180-200 fine, subflexuous, radiating 

 striae,*' is doubtless synonymous with C. Qlinoisensis, and the species 

 would probably never have been described if Hall or Meek and Worthen 

 had stated accurately the number of costae in that species. 



The specimens in the Chonopectus fauna at Burlington, Iowa, which 

 were referred to O. illinoisensis by the writer, 6 are probably a distinct 

 species, characterized by its coarser markings, the costae usually being 

 about 110 to 120 in number on an average specimen, about 4 of which 

 occupy the space of one millimeter. The statement in that place that 

 the shells were marked by 120 to 200 costae was based upon observations 

 made in part upon the Burlington limestone specimens. 



6 Transactions of the Saint Louis Academy of Science, vol. 10, p. 67, pi. 1, fig. 14. 



