INVERTEBRATE FOSSILS OF THE CHESTER GROUP. 



RADIATA. 

 ECHINODERMATA. 



CR IN OIDE A. 

 Genus PTEROTOCRINUS, Lyon and Casseday. 



Asterocrinus, Lyon, 1857. Geol. Report Ky., vol. iii, p. 472 ; (not Minister, 1831). 

 Pterotocrinus, Lyon and Casseday, Jan., 1859. Am. Jour. Sci. and Arts, vol. xxix, p. 68. 

 Generic formula. — 



Basal plates, 2. 



Radials, 1 or 2 — f-5 ; the second minute or sometimes wanting. 



Secondary radials, 1 + 10; small, and easily confounded with the brachial 

 pieces. 



Anal pieces, 1, known. 



Interradials, 4 ; small probably, sometimes wanting . 



Arms, 20. 



Interbrachial appendages, 5 ; very large. 



Body, below the arms, broad and short ; truncated, concave, 

 and often rounded below. Base depressed or discoid. First 

 radial pieces much larger than the second ; two of them rest- 

 ing upon each of the two basal pieces, and the fifth occupying 

 a more or less defined notch at the anterior end of the suture 

 between the basal pieces, so as to rest partly upon each of the 

 latter. Anal piece in the typical species, small, but in others 

 larger, or nearly or quite equaling the first radials, between 



