356 PROCEEDINGS OF THE ACADEMY OF [1888. 



mously developed perisomic plates" (Revision, Pt III, p. 63), and 

 not as true interradials, although they present a more rigid appear- 

 ance than perisomic plates generally have. Our views have been 

 strengthened by De Loriol's important discovery of the plates cover- 

 ing the ventral surface in Apiocrinus roissyanus. ^ According to 

 his description the space between the rays, from the first or the first 

 two interradial pieces up, are occupied by transverse series of tw'O or 

 three small, somewhat regular plates, which gradually lose their reg- 

 ularity, and at the top of the third radial become for the most part 

 entirely irregular and unequal. They differ in their form and 

 arrangement in every one of the interradial spaces, and pass into 

 a conical "ventral sac," which rises to the top of about the ninth 

 brachial piece. The plates composing this ventral covering are 

 equally irregular, and, though tolerably strong, are not absolute- 

 ly rigid. De Loriol considers them as constituting a pliable integu- 

 ment, and not a solid vault like that of Actinocrinus, but in the 

 specimen the central jDortion was not preserved and he could not dis- 

 cover the condition of the mouth, nor could he find traces of the 

 ambulacra. In the same paper, on page 14, De Loriol also describes 

 a specimen of Apiocrhms magmficus, in which the interradial 

 spaces between the third radials, and up to the first brachial piece, 

 are occupied by numerous irregular plates, dissimilar in the different 

 spaces. He considers these interradial plates, in both species as be- 

 longing to the "ventral sac," which was capable, in his opinion, of 

 contraction or expansion. 



A similar irregularity in the interradials exists among the Ichthyo- 

 crinidae. In Ichthyocrinus interradials and interaxillaries are gen- 

 erally wanting, but in the one species in which they have been found 

 their arrangement seems to be rather uniform in the different spaces. 

 In Forhesiocrinus, which also has interradials, we frequently find two 

 plates in the first row at the azygous side, in other cases but one. In 

 Taxocrinns, when the rays are close together, there are sometimes no 

 interradials at all, or, when there are more than one, the first is 

 followed by one or two smaller plates. In Taxomnus Thiemei, the 

 type specimen has neither interradials nor interaxillaries, while other 

 si:)ecimens in our collection, not otherwise distinguishable, have one 

 to three interradials. In Taxocrinns inter scapular is (Iowa Geol. 

 Rep. 1858, Vol. L, Pt II, PI. 1, fig. 3), we find. a single plate inter- 



^ Note sur Quelques Echinodermes Fossils des Environs de la Roclielle. 1887. 

 p. 11. 



