574 PALAEONTOLOGY OF ILLINOIS. 



ascending with a backward slant to the hinge ; beaks located 

 near the middle of the hinge line, above which they seem 

 scarcely to rise. Surface marked by extremely fine con- 

 centric striae, and a few somewhat larger furrows or wrin- 

 kles of growth, crossed on the anterior (?) half of each 

 valve by small radiating costse, generally not denned near 

 the anterior (?) margin. Antero-posterior diameter 0.25 

 inch; hight, or diameter at right angles to the hinge, 0.20 

 inch; convexity unknown. 



Of this litttle shell we have numerous specimens, all compressed to 

 entire flatness on the surfaces of the lamina? 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 l)e 

 different, while their radiating costa? also indicate different affinities. 

 Some individuals are a little wider proportionally, in their antero-pos- 

 terior diameter, than that from which the above measurements 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 de- 

 scribed as the anterior must be the posterior, and vice versa. The rea- 

 sons for doubting its relations to the genus Lima, however, are (1), 

 extreme thinness ; (2), the fact that it seems to have a prismatic struc- 

 ture ; and (3), its very small size. It is possible, however, that the ex- 

 tremely 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 structure, it would 

 more probably belong to some perhaps uudescribed genus, allied to 

 Aviculopecten, of the family Aviculidce. 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 

 belonging to a very distinct group, to which BEYKICH has applied the 

 name Pseudomonotis. 



Locality and position Jacksonville coal shaft, roof of coal No. 3 ? of 

 the Illinois section, Jacksonville, Illinois. 



