NO. 1582. INFllABASALS L\ THE FAMILY PENT AC RINITID.i:— CLARK. 678 



Fig. 3. — Radi.m.s, basals, and infra- 



BASALS OF MeTACRINUS ROTUNDUS. 



most) columnar, which has two of the primitive lobes, which will 

 eventually become the angles of the stem, well developed, while the 

 primitive ring is as yet barely a semicircle. It is noticeable also in 

 stem joints which have the lobes well developed that the ring imme- 

 diatelv around the central canal is always much more dense than the 

 extremely delicate lobes. This points to the conclusion that the 

 pentagonal character of the stem, at 

 least in the Pentacrinitidte, is derived 

 from an ancestral type, in which the 

 stem is composed of circular columnars, 

 as in Encr'miis; for, Avere this not so, we 

 should expect the lobes to be developed 

 at the same time as the inner ring, in- 

 stead of being merely a delicate net- 

 work of delicate calcareous threads 

 when the latter is well developed and 

 composed of a comparatively dense de- 

 posit. 



. The second colunmar in this specimen 

 consists of a calcareous ring, bearing five unequal lobes, of very deli- 

 cate structure, much more delicate than the comparatively solid ring 

 upon which they are borne; the specimen figured by Doctor Carpen- 

 ter" on Plate xxiii, fig. 1, is very similar, but is somewhat more ad- 

 vanced in growth ; the third columnar is similar, but shows a marked 

 thickening all around (see same reference, Plate xxiii, fig. 2), while 

 the fourth has the lobes of almost equal size, and the raised edges of 

 the sectors with the dentate processes are beginning to form (see same 

 reference, Plate xxiii, fig. 3). 



Encouraged by my success in the demonstra- 

 tion of the infrabasals in Isocriniia deeorni.s (Wy- 

 ville Thomson) and Metacrinvx rotund lus P. H. 

 Carpenter, I decided to carry my investigations 

 still further, and to endeavor to point them out 

 in all the species of both genera of which I could 

 obtain material. I also wished to isolate the in- 

 frabasals, if possible, and to determine their size 

 and their relations to the basals. This I did not 

 consider myself justified in doing before, and the specimens figured, 

 therefore, were mounted on glass slides exactly in the state in which 

 they were figured, and have now become part of the collection of re- 

 cent crinoidea belonging to the U. S. National Museum, where they 

 will be available for future study. 



Fig. 4.— Uppermost col- 

 umnar OF Metackinus 



ROTUNDUS. 



« Challenger Reports, XI, Zoology, 1884. 



