GU-STEROPODS. 177 



Capulus obliquus (KEYES). 



Plate liii. figs. 14a-b 

 Platyceras obliquum Keyes, 1888 : Proc. Am. Philosophical Soc., vol. XXV, 



p. 141, figs. 12-13. (Reprint, p. 13.) 



Platyceras obliquum Keyes, 1889: Proc. Acad. Nat. Sci., Phila., p. 290. 

 Capulus obliquus Keyes, 1890: Am. Geologist, vol. VI, p. 9. 

 Capulus obliquus Keyes, 1890 : Proc. Acad. Nat. Sci., Phila., p. 168, pi. ii, 



fig. 3. 



Shell of medium size, irregularly oblong, subspiral, forming 

 one volution ; regularly enlarging, slightly more rapidly trans- 

 versely than in the opposite direction, to the aperture. Apex 

 large, obtuse, far removed from the body of the shell, which is 

 broadly arcuate ; very noticeably oblique to the plane of gen- 

 eral curvature in the body of the shell. Aperture irregularly 

 quadrangular in outline ; margin sharp and more or less sinu- 

 ous. Surface marked by several undefined plications, which 

 sometimes form longitudinal series of obscure nodes; these 

 are crossed by numerous sinuous, often somewhat imbricated, 

 lines of growth. 



Horizon and localities. Lower Carboniferous, Burlington 

 limestone: Hannibal; Burlington (Iowa). 



Capulus biserialis (BALL). 



Plate liii, figs. 12a-b. 



Platyceras biserialis Hall, 1859 : Geology Iowa, vol. I, pt. ii, Supp.,p. 90. 

 Platyceras biserialis Moek & Wortben, 1868: Geol. Sur. Illinois, vol. Ill, 



p. 509, pi. xv, figs. 3a-b. 



Capulus biserialis Keyes, 1890 : American Geologist, vol. VI, p. 9. 

 Capulus biserialis Keyes, 1890: Proc. Acad. Nat. Sci., Phila., p. 167. 



This species is similar to Capulus tribulosus ( White ), but 

 has only two, instead of three, longitudinal rows of spines. 



Horizon and localities. Burlington limestone : Quincy, 

 Illinois. 



A marked characteristic of this form and also of C. tribu- 

 losus ( White ) is, that the tubular spines are arranged in longi- 

 tudinal rows, while in the few other American spine-bearing 

 Capuli, there is no regularity in the distribution of the spinous 

 processes. The spines are easily broken, and hence are seldom 

 preserved to their full length; often they are scarcely notice- 

 able. 



G 13 



