538 NORTH AMERICAN INDEX FOSSILS. 



XLIV. Megistocrinus Owen and Shumard. 

 Usually large. Calyx wider than high, flattened and sometimes 

 excavated at bottom ; plates heavy. B 3, closely anchylosed, form- 

 ing a thick, hexagonal plate. R usually spread horizontally, wider 

 than long and hexagonal. Costals similar to R in form and size. 

 Number of brachials incorporated with the calyx variable. Arms 

 biserial throughout and branching. Pinnules small and rarely pre- 

 served. Tegmen low hemispherical. Anus excentric, sometimes 

 marginal. Column large and long, with five-lobed central canal. 

 Devonic-Mississippic. 



125. M. abnormis (Lyon). Devonic. 

 Dorsal cup saucer-shaped, expanding from the bottom to the 



arm bases which extend out in five large lobes with deep notches 

 between. Plates smooth. Anus a little above the arm bases open- 

 ing through a flat area. 



Onondagan of Kentucky and Indiana. 



126. M. depressus (Hall). Devonic. 

 Calyx wider than high; dorsal cup basin-shaped, flattened at 



bottom and on the sides, but expanding into a short rim for the 

 arm bases. Plates flat, thickened at the margins and covered in 

 well preserved specimens by numerous very fine striae and at times 

 with small pustules. 



Hamilton of New York and Kentucky. 



127. M. spinulosus Lyon. Devonic. 

 Distinguished by rays of eight primary arms each, arranged in 



an uninterrupted line around the calyx. 



Hamilton of Kentucky and Ohio. 



128. M. evansi Owen and Shumard. (Fig. 

 1867.) Mississippic. 



Sometimes very large (varying from one 

 fifth of an inch to two and one half inches 

 Fig. 1867. Megistocri- long). Basi-radial sutures broad and 

 nus evansi, anal view of deeply channelled. Anus at end of a pro- 

 calyx. (After Meek and ^ , -^i.- .t. 

 ^ , . tuberance near or withm the arm regions. 



Arm openings in pairs (five pairs in younger 

 forms and ten in older). (Type of genus.) 

 Burlington of Missouri, Illinois and Iowa. 



