PALAEONTOLOGY OF IOWA. 



481 



of which is as large or larger than the third radial plate ; a 

 third, fourth, fifth and sixth series of smaller interradial 

 plates. First anal plate as large as the first radial plates, 

 supporting one central large plate, and on the outer sloping 

 edges of each side a series of two plates of the same dimen- 

 sions as the second and third radials, while above and within 

 these are several series of smaller plates to the number of 

 more than thirty. 



Dome broad, depressed hemispheric, composed of a great 

 number of small plates with a subcentral proboscis or spine. 



FIG. 53. Diagram of the plates composing the body, below the arms, of Megistocrinus latus. 



This species resembles one in the Hamilton group of New- York, but is larger and 

 the basal plates more extended beyond the column : the form and proportion of the 

 plates also differ in some degree. The surface of each plate is distinctly depressed in 

 the New- York species, and the margins at their junction form an elevated ridge ; 

 while the plates of the Iowa species are individually convex. 



Fig. 1 a. The summit of the specimen. Fig. 1 6. Basal view of the same. 



Geological formation and locality. In the calcareous shales of the age 

 of the Hamilton group of New- York : New-Buffalo, Iowa. 



f IOWA SURVEY.] 



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