INVERTEBRATES. 



231 



Fig 23. 



This is the first species of this ge- 

 nus, we believe, identified in this 

 country, though Prof. Hall has since 

 described a species from the Burling- 

 ton limestone, which he refers to a 

 section of this group. From the de- 

 scription given by him, however, we 

 are inclined to think his species dif- 

 fers too widely from the forms upon 



PaJaichmus burungtonensis. t J ■ L 



Diagram showing a part of two ambulacra, and the which McCoy's genus Was founded, 

 intervening interambulacral pieces, above the middle , i 1 i j • -^ 



of the body ; all magnified near two diameters. to be properly placed in it. 



Locality and position: Burlington group, Subcarboniferous; Burlington, Iowa. 



MOLLUSCA. 



LAMELLIBRANCHIATA. 



Genus AVICULOPECTEN, McCoy, 1851. 



(Ann. Mag. Nat. Hist., vol. vii, p. 171.) 



Aviculopecten burlingtonensis, M. and "W. 



PI. 16, Fig. la and 16. 



Aviculopecten burlingtonensis, Meek and Worthen, Oct., 1860. Preceed. Acad. Nat. 

 Sci., Philad., p. 453. 



Shell of about medium size, very thin and fragile, subcir- 

 cular, slightly wider than long; pallial margin regularly 

 rounded lateral margins; more narrowly rounded near the 

 middle, thence converging towards the beaks at an angle of 

 about 100°; hinge straight, between two-thirds and three- 

 fourths as long as the transverse diameter of the valves, and 

 ranging at right angles to the longitudinal axis of the shell. 

 Left valve much compressed; anterior ear of moderate size, 

 flat, subtriangular, the extremity being a little rounded — sepa- 

 rated from the margin below by a nearly rectangular notch; 



