DESCRIPTION OK A NEW CRINOID. 95 



DESCRIPTION OF A NEW CRINOID FROM THE 

 HAMILTON GROUP OF MICHIGAN. 



nv CHARLES WACHSMUTH. 



Megistocrinus concavus, Nov. Sp. 



Plate I. — ftg'. S. Dorsal aspect of a siiecimen. 



J^ig". 6. Ventral aspect of another specimen. 

 J^ii,''. 7. Profile view. 



This interesting species represents the most extravagant form of 

 Megistocrinus so far discovered. The dorsal side is more depressed 

 than in any other species of this genus; the ventral side highly con- 

 vex, or subglobose, as high or higher than the dorsal side, slightly 

 bulging posteriorly. Fully one-half of the dorsal side, as far as the 

 third or fourth ring of plates, to the third primary radials, is placed 

 within a deep concavity. In most of the s])ecimens, this whole con- 

 cavity is covered by matrix, and it appears as if the base were com- 

 posed of twelve or nineteen pieces, according to whether the second 

 ring only, or the third one also, takes part in the concavity. The 

 u[)per portions of the dorsal side are spread out almost horizontally, 

 and form a shallow cup, which is covered ventrally by a depressed vault, 

 composed of small irregular plates. The basals and first primary radials, 

 which are arranged almost horizontally, and form the deei)est part of 

 the concavity, are scarcely convex at their surface, while all succeeding 

 plates, up to the first secondary radials, inclusively, are extended into 

 a rounded knob. The knobs, which stand out very prominently, cover 

 the entire surface of the i)lates, and give to the specimen an uncom- 

 monly rough appearance. The succeeding plates of the dorsal side, 

 close to the arm bases, and the interradials of the ventral side, are pi'o- 

 vided at the center with a small tubercle, while the central plate, the 

 proximal pieces, and the principal radial dome-plates are larger and 

 highly convex. Another most remarkable feature is presented by the 

 fact that the third or bifurcating radials are larger than the first and 

 second plates, and that the first secondary radials are frequently the 

 largest plates of the calyx. 



Like all species of this genus, Megistocrinus concavus has three large 

 basals of equal size; 3x5 primary radials, and the first anal plate is 



