26 



which is narrow, and slightly open at the base ; nine or ten simple, 

 rounded plications on each side of the sinus, which is shallow, and 

 contains two slightly raised plications. Dorsal valve subelliptical in 

 outline, depi'essed convex, a little flattened at the cardinal extrem- 

 ities ; eight or nine plications on each side of the mesial fold, which 

 is very slightly elevated, and contains three plications, which are 

 a little larger than the others, the middle one being the largest, 

 and sometimes a little flattened near the front. The front border of 

 the mesial fold in the older shells is bent abruptly upward, giving an 

 emarginate appearance to the front of the shell. 



The plicated mesial fold and sinus, the subelliptical outline of the 

 dorsal valve, and the pointing obliquely backward of the beak of the 

 ventral valve, are the more prominent features of this species, and by 

 which it may be readily distinguished. 



Genus Amboc(elia Hall. 



Amboccelia (Spirifer?) minuta (n. s.) Shell very small ; hinge 

 line a little shorter than its greatest width ; outline of dorsal valve sub- 

 circular, depressed convex, most elevated near the umbo, flattened at 

 the front margin ; beak small, projecting a little back of the hinge line ; 

 ventral valve much elevated, arcuate ; beak broad, obtuse, incurved ; 

 foramen about as wide as high, its sides sloping with a slight outward 

 curve ; area indistinctly defined. Surface covered with numerous 

 fine set^, which, when removed, leave a pustulose appearance to the 

 surface, yet the shell structure appears to be fibrous. 



I have not seen a specimen of this species exceeding a line in 

 transverse diameter. 



Locality and ^^osition, in limestone equivalent to the Chemung beds 

 at Burlington, Hamburg, 111., and Hannibal, Mo. 



I am indebted to B. J. Hall, Esq., of Burlington, Iowa, for a single 

 specimen of a shell, the only one I have seen, except some fragments, 

 which cannot be satisfactorilj' referred to any established genus, and 

 for which I propose the generic name of Acambona. The figures ac- 

 companying the generic description are restorations only in part of 

 the outline, as the specimen has been somewhat eroded, but fortu- 

 nately preserves every external generic character, besides showing 

 one of the internal spires. Following the specific description of this 

 shell, is the specific description of another, which may prove to be- 

 long to this genus, when more perfect specimens have been obtained. 

 My cabinet also contains a fragment, specifically different from either 

 of these, which must either be referred to Relzia, or to the genus 

 proposed. 



