652 GEOLOGICAL SURVEY OF IOWA. 



Fig 3. A portion of the axis of a specimen of this species, showing the usual charac- 

 teristics, together with several of the small props or braces extending from 

 one volution to the other. 



Fig. 4 a. A fragment of the axis preserving several volutions of the frond, as seen from 

 the inner side. 



Fig. 4 b. A portion of the inner celluliferous surface enlarged, showing arrangement of 

 cellules. 



Fig. 5 a. Exterior of the same specimen, showing the fenestrules. 



Fig. 5 6. Enlargement of surface of fig. 5 a, showing form of fenestrule. 



Geological formation and locality. In the Warsaw limestone : Warsaw, 

 Illinois. 



Archimedes reversa ( n. s.). 



PLATE XXII. FIG. 2. 



Axis strong, expanding or thickening towards the middle, 

 and gradually tapering above : volutions of the spiral sini- 

 stral, about four in the space of an inch ; frond unknown. 



This species has the general form and robust aspect of the preceding; but the 

 fusiform character and reversed direction of the spiral frond indicate a probable 

 specific difference, though the latter character not unfrequently exists in the species 

 that are usually dextral. 



In two specimens of the species from the Kaskaskia limestone, where the axis has 

 been apparently broken, the growth has been resumed in a dichotomizing form; one 

 branch being dextral and the other sinistral in its spire. 



Farther observation is necessary to determine the question of identity or difference 

 between this form and the A. wortheni in the same rock. 



Fig. 2. The axis preserving the bases of the frond, showing gradual expansion towards 

 the middle and diminution above. 



Geological formation and, locality. In the Warsaw limestone : Warsaw, 

 Illinois. 



GENUS PTYLOPORA ( SCOULER in MS.) ; 



M'CoT, Synopsis Garb, fossils, 1844. 



" GENERIC CHARACTER. Flabelliform or infundibuliform, 

 attached by roots from which a strong midrib arises, giving 

 origin on each side to thin equidistant interstices connected 

 by regular dissepiments : external face of the interstices 

 carinate, and bearing two rows of pores." 



