228 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



purposes it must be classed as naked ; but it is rough to the touch, and Carpenter 

 noticed that when touched with acid the disk of Pontiometra andersoni effervesced 

 strongly, demonstrating the abundance of lime in its composition. 



In the Himerometridse (figs. 713-718, p. 346) certain species of AmphiTnetra, 

 as A. ensifer and A. schlegetii, have the cutis of the disk filled with small rounded 

 plates resembling those seen in Pentametrocrinus semperi (fig. 1158, pi. 25), while 

 in Himerometra (figs. 713-715, p. 346) larger and more scattered concretions much 

 like those present in certain comasterids are sometimes found situated in the deeper 

 layers. 



The relationships between the several types of disk and the various systematic 

 groups is summarized in the following table : 



A. Naked. 



B. Small interradial plates only. 



C. Complete dorsal interradial plating, but no ventral plates. 



D. Scattered thick nodules. 



E. Very numerous, thin, noncontiguous plates, not projecting above the general 

 surface of the disk. 



F. Larger, thicker, conspicuous, projecting, and, especially along the ambu- 

 lacral grooves, more or less contiguous plates. 



G. A complete perisomic plating. 



a. Disk entire. 



b. Disk incised. 

 Comasteridse, A, B, C, D, a. 

 Zygometridae, F, G, a, b. 

 Himerometridse, A, D, E, a, b. 

 Stephanometridse ; Atelecrinidae, A, b. 

 Tropiometridse, A, a. 

 Mariametridse, A, F, G, b. 

 Colobometridse, A, F, G, a, ~b. 

 Calometridse, G, a. 

 Thalassometridse, F, b. 

 Charitometridse, F, G, b. 

 Antedonidse, A, B, D, a. 

 Pentametrocrinidse, A, E, b. 



.s'irfe anil eorcring plates. 



The perisomic skeleton of the arms and pinnules in the comatulids was first 

 noticed in 1866 in Antedon bifida (pi. 50, fig. 1335) by W. B. Carpenter; in this 

 species, however, it disappears before the adult stage is reached. 



In 1868 Michael Sars described the characteristic rods in the ambulacral 

 lappets of Hathrometra sarsli (fig. 1174, pi. 27). 



In a semipopular article published in 1880 P. H. Carpenter figured highly 

 developed side and covering plates comparable to those of the pentacrinites which 

 he had found in a new comatulid (subsequently called accela) dredged by the 



