INVERTEBRATES. 255 



wider than long, (other examples a little longer than wide); 

 greatest convexity at the middle ; valves equally gibbous ; 

 lateral margins usually prominent and narrowly rounded in 

 outline at the middle, thence converging with a slightly con- 

 vex outline to the faintly subtruncate front ; postero-laterai 

 margins a little inflected, so as to form slight-concavities, con- 

 verging to the beaks at an angle of about 97°. Both valves 

 destitute of a mesial fold or sinus, but each sometimes slightly 

 flattened in the middle near the front, where they meet with- 

 out the slightest sinuosity in the margin of either. Beak of 

 ventral valve small, pointed, closely incurved upon that of the 

 other valve, which is but little less prominent, foramen round 

 and very small. Surface with small, obscure, concentric marks 

 of growth. Spiral appendages each making^ about twelve 

 turns. Surface (probably exfoliated) showing only small, 

 obscure, concentric ridges. Breadth of a rather wide specimen, 

 0.84 inch; length of do., about 0.77 inch ; convexity, 0.52 inch. 



Since publishing a description of this form under the name of Athyris par- 

 virostris, in the proceedings of the Philadelphia Academy of Sciences, further 

 comparisons with some additional specimens, lead us to think it most probably 

 only a variety of A. planosulcata, of Phillips, to which we now refer it provis- 

 ionally. Nearly all the specimens we have yet seen, however, differ from the 

 figures, as well as from authentic English examples of Phillips' species, with 

 which we have compared them, in being much less nearly circular, or more 

 nearly subquadrate in'outline. Possibly a more extensive series of specimens 

 may prove it entitled to rank as a distinct species, but with our present means 

 of judging, we incline to the opinion that the differences observable, are not of 

 specific value. 



None of the specimens we have seen show the projecting, concentric lamellae 

 of growth so characteristic of the A. planosulcata ;*though as they were all 

 broken from a hard limestone, and seem to have lostthe outer layers of the 

 shell, we could scarcely expect any traces of the projecting lamillae to be retain- 

 ed under such circumstances. Hence their absence is no evidence that they 

 may not have existed. 



Locality and position: Keokuk division of Subcarboniferous series; near War- 

 saw, Illinois. 



