1881.] NATURAL SCIENCES OF PHILADELPHIA. 327 



body plates, includiug dome, covered with peculiar granules or 

 indistinct wrinkles. 



Basals three, moderately large, forming together a shallow 

 hexagonal basin, the upper side without re-entering angles ; a low 

 rim borders the columnar facet. Primary radials 3X5; the first 

 wider than high, much narrower at their junction with the basals, 

 superior lateral sides short ; second radials generally shorter than 

 the first, but wider ; third radials wide, as high as the second ; the 

 second are arranged horizontall}^ to the vertical axis of the bodj^, 

 while the third sometimes take even a downward direction. In 

 the European species only the secondary radials assume that 

 position ; in American species the convexity of the calix does 

 not extend beyond the first radials. The upper radials bend so 

 abruptly outward, that only their lower corners come in contact 

 with the interradials, the plates themselves form the base of a 

 lobed lateral extension ; contrary to Agaricocrinus, in which they 

 are flattened out, and form a disk in connection with the adjoining 

 pieces. 



Secondarj' radials 1 X 10, either all bifurcating, or one side of 

 each ra}^ only. In the former case, each plate supports on each 

 of its sloping sides a tertiarj' radial, which in turn supports the 

 primary arms ; in the other, one arm is given off from a secondarj- 

 radial. The rays, to the base of the arms, are spread out hori- 

 zontally, and the arms curve upward very gradually ; this gives 

 to the specimen, with the arms attached, an unusually broad 

 appearance, its width and length being almost equal. Arms 

 cylindrical, strong, of nearly equal size to their full length, 

 divergent, simple or ramifying, composed of two series of very 

 narrow pieces, alternately arranged, rounded at the back, with 

 zigzag suture lines. In A. spinohrachialus Hall, in which the 

 arms on becoming free remain simple, some of the joints at each 

 side of the arm are extended into long lateral spines, which stand 

 out conspicuously from both sides of the arm bases. The lower 

 ones are short, but they increase in length upward until they 

 attain a length of one and one-half the width of the arm. The 

 spines are placed farther apart as they increase in size, the 

 distance being lengthened each time by one additional joint. In 

 species in which the arms divide, the joints are smooth, and the 

 bifurcation takes place from the double-jointed arms, as in the case 

 of Periechocrinus^ not from single plates as in Platycrinus, etc. 



