672 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXIII. 



Fig. 1.— K a dials , 

 basals, and in- 

 feabasals op iso- 

 ceinus d e c o b u s 

 (a young speci- 

 MEN). 



Two specimens of Isocrinus decorus were dissected, one being a 

 small, immature example, the other full grown, and apparently 

 mature. 



In the former (fig. 1) the infrabasals are five small rounded plates, 

 quite distinct from each other, set close together in a fiv^e-lobed 

 rosette in the middle of the star-shaped figure formed by the basals, 

 exhibiting practically the same condition as in the 

 S2:)ecimen of /. leuthardl figured by de Loriol." Their 

 protrusion beyond the dorsal surface of the basals is 

 relatively great; they have a strongly convex dorsal 

 surface, and show no tendency toward degeneration 

 or resorption, as is the case with the quinquelobular 

 rosette representing the fused infrabasals of Cala- 

 mocrinus diomeda\ In the figure only the two infra- 

 basals in the lower and lower left-hand portion are 

 shown entire, the others being more or less covered by 

 portions of the upper stem joints, which could not 

 be removed without risking the specimen. 



In the adult example figured (fig. 2) the infrabasals are flatter, 

 and appear as roughly triangular plates, with a marked depression 

 forming a notch in the outer edge, the rounded ridge on each side 

 of this depression being a continuation of the lateral ridges on the 

 basals, which bear the dentate processes. 



In Metacrinus rotundus (fig. 3) the infrabasals are rounded tri- 

 angular plates, with a flatter dorsal surface than in Isocrinus decorus^ 

 so that their dorsal surface is even with that of the 

 basals. As in the adult specimen of Isocnnus de- 

 co?nis, the petaloid markings are continued onto them, 

 in the form of a prominent U-shaped ridge. 



The topmost columnar of this last specimen ap- 

 pears to be much 3^ounger than any heretofore noticed 

 in the Pentacrinitida^ (fig. 4) ; it is composed of a deli- 

 cate calcareous network, approximately semicircular, 

 bearing two radiating lobes of unequal size, composed 

 of exceedingly delicate calcareous meshes. It was 

 unfortunately impossible to discover the orientation 

 (in reference to the plane of symmetry of the disk) of the larger lobe. 

 Judging from the condition of this stem joint, the columnars arise 

 at a point close to the axial cord, the growth being in each direc- 

 tion around (and close to) the central opening; when this grow- 

 ing calcareous band reaches the median line of a basal, a radiating 

 lobe is formed which increases very rapidly in height, build- 

 ing u]) with a much more open structure than the original 

 band. Thus we get the condition shown by this first (i. e., top- 



FlG. 2.~Radials, 

 BASALS, AND I N - 

 FRA BASALS OF ISO- 

 CRINUS DECORUS 

 (A MATURE speci- 

 men). 



" Revue Suisse de Zool., II, 1894, pi. xxiv, fig. 12o. 



