516 PALAEONTOLOGY OF ILLINOIS. 



MOLLUSCA. 



G-ASTEROPODA. 

 GENUS PLATYCERAS, Conrad. 

 PLATYCERAS UNCUM, M. and W. 



PL 17, Fig. 1. 

 Platyceras uncum, MEEK and WOKTHEN, 1866. Proceed. Acad. !N"at. Sci.. PMla., p. 264. 



SHELL, under medium size, in adult examples elongate- 

 conical and oblique ; body portion nearly straight, especially 

 on the posterior side ; apex attenuate, pointed, laterally 

 compressed and curved backwards (without any lateral 

 obliquity) so as to form a free hook of about half a turn. 

 Aperture generally a little wider transversely than the ante- 

 ro-posterior diameter, and usually showing a faintly suban- 

 gular outline, produced by the prominence of the front and 

 the flattening of the posterior side of the body. Lip irregu- 

 larly undulated, prominent on each side, broadly sinuous 

 behind, and provided with a very deep, narrow sinus. 

 Surface with the usual undulating lines of growth, crossed 

 on the lower half of the body by small, rather obscure, lon- 

 gitudinal plications, and in front by a larger but narrow, 

 prominent ridge, upon which the lines of growth make a 

 strong curve, indicating the presence of the anterior sinus 

 during most of the growth of the shell. 



Length, 1 inch ; breadth (transverse diameter of the ap- 

 erture,) 0.70 inch; antero-posterior diameter of the aperture, 

 0.55 inch. 



This species is intermediate in size, as well as in some other respects, 

 between Platyceras acutirostris (=Capulus acutirostris, Hall, and P.cequi- 

 latera, Hall.) In size and general appearance, it agrees most nearly 

 with the former, though it is larger and differs in having its ap<-x 

 merely hooked instead of subspiral, as well as in its prominent anterior 

 ridge and deeper and narrower anterior sinus. From P. crquilatera, 



