1885.1 NATURAL SCIENCES OF PHILADELPHIA. 295 



Palreozic Crinoids, and he, therefore, does not attach to these 

 plates the value which we think they deserve. According to our 

 interpretation they are present in all Palreocrinoids, but absent 

 or incompletely developed in the Neocrinoidea. By means of 

 the interradials the two groups differ essentially in their larval 

 state ; the whole ventral surface of the Neocrinoid larva is 

 covered by the orals, but in the Palseocrinoid larva the inter- 

 radials physiologically take their place, and the orals or their 

 equivalent is subtegminal. The indistinct calyx interradials, 

 which appear for a short period in the Pentacrinoid larva, be- 

 came resorbed before taking any prominent part in the formation 

 of the calyx, while the interradials of all Palsocrinoids are well 

 defined and permanent plates. It is possible that the interradials 

 of the Encrinidae were similarly resorbed shortly before the 

 Crinoid reached maturity, but they were evidently well developed 

 in their earlier life, as we may judge from their affinities with the 

 C} T athocriniclae and Poteriocrinida?, and this, principally, has in- 

 duced us to refer them to the Palaeocrinoidea. 



We propose the following definitions of the two groups : — 



PAL^EOQRHSTOIDEA Wachsmuth. 



Crinoids with irregularty pentamerous calyx ; plates united by 

 suture or articulation. Base monocyclic or dicyclic. Basals and 

 underbasals variable in number. First radials rarely in lateral 

 contact all around, two of them often separated by an anal plate, 

 and sometimes all of them by interradials. The succeeding plates 

 of the rays are free or become incorporated into the calyx. Arms 

 more frequently biserial than uniserial. There is always at least 

 one interradial to each side which is located ventrally, but when 

 there are a number of them, dorsally and ventrally. The interra- 

 dials extend to the summit plates or cover them, occup\ T the 

 greater portion of the ventral surface, and either form a vault 

 over the perisome or support the perisome ; in either case, how- 

 ever, mouth and disk ambulacra are completely closed. The 

 summit plates are substantially a repetition of the plates in the 

 calyx. They consist of an undivided plate which represents the 

 basals ; of the proximals or interradials and anals ; and frequently 

 of radial dome plates. 



NEOCRINOIDEA Carpenter. 



Crinoids with regularly pentamerous calyx, without interradial 

 or anal plates (Thaumatocrinus excepted). Underbasals rarely 



