85 



Family ACTI IMOCRI N I DAE. 



MEGIST0CRIN17S EXPANSUS n. Sp. 



Plate III, Fig. 18, basal view of a large specimen; Fig. 19, sum- 

 mit view of the same; Fig. 20, side view of the same; 

 Fig. 21, basal view of a, small specimen 

 showing the commencement of 

 the horizontal arms. 



Calyx very shallow; broadly basin-shaped; from three to five 

 times as wide as high; columnar cavity, evenly concave, commenc- 

 ing from about the middle of the second radials, the superior part 

 of which curve upward; the calyx continues to expand from the 

 second radials to the arms, expanding more rapidly as the arms 

 are approached. The arms are directed horizontally. The column 

 is round and of medium size. The vault is one-half higher than 

 the calyx and has twice the capacity. 



The basal plates have an hexagonal outline, about one-half wider 

 than the diameter of the column. The first primary radials abut- 

 ting on a single basal plate are hexagonal, those abutting on two 

 basals are heptagonal. The second primary radials are a little 

 larger than the first and hexagonal. The third primary radials 

 are about the size of the first, pentagonal, and support upon each 

 upper sloping side secondary radials. 



The external surfaces of all the plates covering the three speci- 

 mens at hand, one of them being intermediate in size between the 

 two illustrated, are more or less eroded or disintegrated. The 

 best preserved plates are beveled at the sutures, and it is, there- 

 fore, believed that specimens having a well preserved external sur- 

 face will show distinctly the outlines of all the plates. In our 

 specimens the sutures of the secondary and tertiary radials and 

 those of the vault are generally obscure; some of them are, how- 

 ever, distinct. This is the reason the illustrations are not made 



