38 PROCEEDINGS OF THE ACADEMY OF 



to have been smooth, or only marked with the concentric strias, but internal 

 casts of large individuals sometimes show very faint traces of a few broad ir- 

 regular, radiating, flattened ridges. 



It is probable that this species is most nearly allied to Stricklandinia David- 

 soni, of Billings (Geol. Mag. vol. v, pi. iv, fig. 1, 1 a), which, in some stages 

 of its growth, it resembled rather nearly in form. In all the large examples, 

 however, it differs extremely from that shell, in its remarkable narrowness 

 across the umbones, and its truncated or flattened posterior lateral margins. 

 Its front is also less produced aud less narrowly rounded in the middle in 

 these larger specimens. 



Locality and position. All the specimens of this species we have seen were 

 found loose in Carroll County, Illinois, near rocks of the age of the Niagara 

 group. They are all in the condition of white quartz casts of the interior. 



LAMELLIBRANCHIATA. 



MONOTIS ? GREGARIA. 



Shell very small, extremely thin, compressed, oblique, varying from trun- 

 cato-suboval to subcircular ; hinge line less than the breadth of the valves ; 

 auricles small, obtusely angular, undefined by any sinuosity of the margins ; 

 posterior (?) margin rather regularly rounded in outline, and rounding into 

 the pallial margin ; anter. (?) border prominent below and rounding into the 

 base, but straight and ascending with a backward slant to the hinge ; beaks 

 located near the middle of the hinge line, above which tbey seem scarcely to 

 rise. Surface marked by extremely fine concentric striae, and a few some- 

 what larger furrows or wrinkles of growth, crossed on the anterior (?) half 

 of each valve by small radiating costs?, generally not defined near the an- 

 terior (?) margin. 



Antero-posterior diameter, 0-25 inch ; height, or diameter at right angles 

 to the hinge, 0-20 inch ; convexity unknown. 



Of this little shell we have numerous specimens, all compressed to entire 

 flatness on the surfaces of the laminae of shale, many of them lying with the 

 two valves opened out and connected by their hinge margins. As thus seen, 

 their small size causes them to appear much like the valves of Posidonomia, 

 or those of some of the little phyllopod Crustacea. This form, however, is 

 found, on closer examination, to be different, while their radiating costaa also 

 indicate different affinities. Some individuals are a little wider proportionally, 

 in their antero-posterior diameter, than that from which the ab'ove measure- 

 ments were taken, and these have much the outline of the left valve of some 

 forms of Aviculopecten, excepting that the auricles are not in the slightest de- 

 gree defined (in either valve) by any traces of a marginal sinus. 



It is possible that this little shell may be a true Lima, as it has much the 

 form of some species of that genus, and there certainly are in > the Western 

 Coal-measures, two or more species apparently agreeing in all respects with 

 that genus. If a Lima, of course the side we have described as the anterior 

 must be the posterior, and vice versa. The reasons for doubting its relations 

 to the genus Lima, however, are, (1), its extreme thinness; (2), the fact that 

 it seems to have a prismatic structure ; and (3), its very small size. It is 

 possible, how-ver, that the extremely thin fibrous shell, as we now see, 

 may consist only of the external lamina, left after the decomposition of the 

 inner layers. If so, and the fibrous appearance is really the original struc- 

 ture, it would more probably belong to some perhaps undescribed genus, 

 allied to Aviculopecten, of the family Aviculidaz. If a true Monotis, it would 

 be the only known species of that genus in our Carboniferous rocks, the 

 common Western Coal-measure shells usually referred to that genus, belong- 

 ing to a very distinct group, to which Beyrich has applied the name Pseudo- 

 mon 'tis. 



L'cdity and position. Jacksonville Shaft, Illinois, from near the middle of 

 the Coal-measures. 



[April, 



