42 PAL.550NTOLOGY. 



In naming a Sjjirifer after my friend Heuiy Engelmann, esq., in the 

 Illinois Geological Report (vol. 3, p. 398), I had forgotten that the 

 above-named species had been previously dedicated to the same gentleman. 

 This renders it necessary to find another name for the Illinois species. I 

 would therefore propose to designate it as Spirifer Wortheni. 



Locality and position. — Same as last. 



Spirifer (Tri^onotreta) argentarius. Meek. 



Plate 3, figs. 4, 4 a, and 4 6. 



Shell rather small, moderately convex, wider than long, and having a 

 general subsemicircular or subtrigonal outline, with the greatest breadth on 

 the hinge-line; lateral extremities acutely angular; valves nearly equally 

 convex. Ventral valve with the greatest convexity between the middle 

 and the umbo; beak strongly incurved; area rather low, with nearly par- 

 allel sides near the break, but somewhat abruptly narrowed at the lateral 

 extremities, though continued the entire length of the hinge; foramen wider 

 than high; mesial sinus shallow and narrow, but well defined by the mar- 

 ginal rib on each side extending quite to the beak, and without costae; 

 lateral slopes each occupied by from twelve to fourteen simple radiating 

 costaj, which diminish very gradually in size toward the lateral extremities. 

 Dorsal valve most convex near the middle; beak rather distinctly incurved; 

 mesial fold corresponding in size to the sinus of the other valve, being 

 rather low, and flattened on top along its whole length, with a more or less 

 defined furrow along its middle; lateral slopes costate, as in the other valve. 

 Surface of both valves marked with very fine, regular, undulating lines of 

 growth, most distinct between the costse. 



Length, 0.55 inch; bi-eadth, 0.80 inch; convexity, 0.44 inch. 



It is hardly possible that this can be a variety of the last; at least, with 

 the means of comparison now available, it certainly seems to be quite dis- 

 tinct. It is true I have only two specimens of this form, but of the other 

 we have a sufficient number of individuals, all agreeing with each other, 

 and differing from this to such an extent as to render it very improbable 

 that there may be connecting forms. On comparison, this species will l)e 

 seen to differ very decidedly in having a much narrower (lower) and more 



