484 GEOLOGICAL SURVEY OF IOWA. 



GENUS PENTREHITES (SAY). 



FIG. 54. GENERIC FORMULA. 



Basal plates 3, of which one is smaller than the others. 



Radial plates 1X9, forked, large. 



Interradial plates 1X5, lanceolate, small. 



Pseudo-ambulacral areas 1X5, covered by pinnules. 



Mouth 1, central. 



Anal aperture 1, lateral. 



Ovarian apertures 2X5, arranged around the mouth. 



"BODY variable, globose, ovoid, pyriforni, or clavate : 

 basal plates three, one smaller and quadrangular, the other 

 two equal in size and pentagonal ; radial plates five, equal 

 in form and size, two resting each on one of the sides of the 

 two pentagonal plates, while the three others are alternate 

 and in contact each with two of the basal plates. The little 

 quadrangular basal plate does not correspond, as might be 

 supposed, with the anal side, but with a regular side, as in 

 PLATYCRINUS. These radial plates are divided in the middle 

 by a sinus more or less wide and deep, giving them the 

 form of a two-pronged fork : this sinus contains the pseudo- 

 ambulacral areas. The forks of the radial plates alternate 

 with five little interradial pieces, which are lanceolate, 

 subtetragonal, more or less elongate, and meeting at the 

 superior central opening. 



" The pseudo-ambulacral areas are composed of three series 

 of plates superimposed upon each other. The middle is filled 

 with a series of compressed tubes disposed parallel to the 

 sides of the forks of the radial plates, and succeeded by a 

 single median lanceolate or linear plate, ornamented on each 

 side by a great number of little transverse poral plates, al- 

 ternating with transverse groups of still smaller plates (sup- 

 plementary poral pieces). Filiform and articulate appendages, 

 like the pinnules of the arms of other crinoids, and corre- 



