PALAEONTOLOGY OF IOWA. 685 



" COSTALS five, subhexagonal; 

 "SCAPULARS five, pentagonal; 

 " ARMS five, pentagonal." 

 The above description of this genus is from the MS. of Dr. TROOST. 



The genus has been recognized by Drs. D. D. OWEN and SHUMARD in the Report 

 on Wisconsin, Iowa and Minnesota, p. 597; and by Dr. SHUMARD in the Geological 

 Report of Missouri. It may be characterized according to the nomenclature here used, 

 as follows : 



FIQ. 113. 



GENERIC FORMULA. 

 Basal plates, 5. 

 Subradial plates, 5. 

 Radials 2 X 5 = 10, being 2 from each 

 ray*. 



Anal plates, 4 or more? 



Agassizocrinus dactyliformis (TROOST ). 



The basal plates are often prominent, forming together a mamilliform protuberance 

 which projects beyond the subradial plates. The cicatrix for the attachment of a 

 column is usually null, or very obscure. The subradial plates are large and prominent, 

 sometimes gibbous. The radial plates are small, pentagonal; the first one wedgeform 

 below, and the second wedgeform above. The arms are composed of a single series of 

 short quadrangular plates, which are scarcely longer on one side than on the other : 

 they are alternately tentaculate, the tentacula being composed of short strong joints, 

 which, in section, are like the arm-joints, semicircular or semielliptical. The anal 

 side is conspicuously marked by the presence of four plates arranged as in POTKRIO- 

 CRINUS ( this number may vary to less or more). 



The structure of the body of the typical species, and others of this genus, does 

 not differ from that of Poteriocrinus, Zeacrinus or Scaphiocrinus . The conspicuous 

 features are the form of body, protuberance of basal plates, and usual absence of a 

 cicatrix for the attachment of a column. The basal plates are extremely thickened, 

 forming together a solid mass with occasionally a scarcely visible foramen extending 

 from the base of the visceral cavity, which sometimes reaches a little below the bot- 



* The second radial plates are described as the arms [arm-plates] by Dr. TROOST, of 

 which there are five, each in fact giving origin to two arms. 



