PLATYCEINID^. 681 



longing to one species. In all of tliem there appear around the stem three 

 conspicuous nodes, which are continued to the upper edges of the basal disk; 

 and in all may be seen the unique articulation between the lower brachials 

 and arm plates ; while the inflation of the arms at their middle portions is a 

 frequent character in mature specimens. But far more striking are the dif- 

 ferences caused by growth. Comparing the smaller specimens with the 

 larger ones, it will be found that the arms in the smallest or youngest ones 

 are not only uniserial throughout, but decidedly zigzag, and that their joints 

 are proportionally much longer than those of the adult. In more advanced 

 specimens the tips of the arms are biserial, and the biserial part extends 

 downward, as the specimens grow larger, by the introduction of additional 

 joints at the top, and the progressive interlocking of the cuneate plates. 

 The modifications here exhibited, and due bej'ond all question to individual 

 growth alone, are so remarkable, that without a knowledge of the ontogen}- 

 of the Crinoids, the earlier stages of this species might readily be taken as 

 representatives not only of different genera, but even of different groups. 



Platycrinus boonvillensis S. A. jiiller. 

 Plate LXXII. Figs. 8a, h, and 9. 



1S91. S. A. MiLLEK ; Bull, i, Geo]. Surv. Missouri, p. 8, Plate 1, Jigs. 1 and 2. 



A rather large species of the type of P. hurlingtonensis. Calyx to the 

 top of the radials bowl-shaped, wider than high, slightly pentagonal as seen 

 from above. Plates moderately heavy, the surface smooth, or nearly so ; 

 the basi-radial and interradial sutures grooved. Basals closely anchylosed, 

 the lines of xmion elevated into ridges ; they form a low, rapidly spreading 

 basin, distinctly pentangular at the upper end, broadl}^ truncated at the 

 lower, the bottom deeply excavated so as to form a rounded, rugose rim 

 around the column. Column facet circular, occupying one half of the width 

 of the concavity, its face covered with radiating strise. Eadials a little wider 

 than long, gradually expanding upwards, slightly more elevated along the 

 median line, and somewhat beveled toward the sutures. The facets occupy 

 less than half the width of the plates; they are shallow, directed upwards, 

 and surrounded by a projecting rim. 



Horizon and Locality. — Lower part of Warsaw limestone ; Boonville, Mo., 

 and Spurgen Hill, Ind. 



Types in the collection of Mr. Sampson. 



