28 



the same as from the apex to the points, distinctly defined, with a 

 flange-like projecting border on each side, and presenting a neatly- 

 curved outline, as seen in profile. Surface marked by strong lines of 

 growth near the front, and by numerous, rounded, slightly raised 

 costa3, which become indistinct on the upper part of the shell, particu- 

 larly on the ventral beak, at the sides, — on the former, scarcely 

 appearing at all, and on the latter assuming the character of fine 

 striae. 



Locality and position, in the lower division of the Burlington Lime- 

 stone, Burlington, Iowa. 



Genus Retzia King. Acambona Mihi. 



Eetzia C Ac AMBON a ? ) ALTiROSTRis (n. s.) Shell sub-ovoid in out- 

 line, valves nearly equally convex. Dorsal valve most convex near 

 the umbo ; beak prominent, small ; beak of ventral valve strong, much 

 elevated, incurved. Area about as high as wide. 



Surface marked by from twenty to twenty-two prominent, simple, 

 rounded ribs on each valve, which gradually enlarge towards the 

 front, the spaces between which are narrower than the ribs, the 

 central one on the ventral valve being a little wider than the rest, 

 and giving the appearance of a faint, narrow, mesial sinus ; the rib on 

 the other valve, immediately opposite, is also a httle broader than the 

 rest, but scarcely raised above them. The only specimens obtained 

 are casts in fine-grained sandstone ; two of these seem to show indi- 

 cations of an apical foramen, yet the direction of the incipient costse 

 on the ventral beak would seem to lead to the conclusion that it was 

 pointed. This, with the great elevation of the ventral beak, the out- 

 line of the area, and its apparent extension forward of the dorsal 

 beak, renders it not improbable that this species properly belongs to 

 the genus Acambona. 



Locality and position, in the sandstone of the Chemung beds at 

 Burlington, Iowa. 



Genus Streptorhynchus King. 



Streptorhynchus lens (n. s.) Shell broadly subelliptical, a 

 little wider than long ; hinge line not quite equalling the greatest 

 width of the shell, but sometimes extended outward, forming salient 

 angles at the cardinal extremities; valves subequally convex, the 

 ventral valve deepest, somewhat regularly convex ; beak prominent, 

 usually incurved, and not twisted or flattened ; area rather narrow, 

 extending fully to the cardinal extremities; height of the callosity a 

 little less than the breadth of its base ; dorsal valve a little flattened 

 at the cardinal extremities, usually having a sUght central depres- 

 sion, producing a slight emargination of the front. 



Surface marked by numerous, abruptly raised, somewhat rugose, 



