428 PELMATOZOA. 



which the anal interradial is not intercalated. The succeeding radials 

 are embraced in the free arms, and do not form part of the calyx. The 

 arms are numerous and bifurcated, and all the divisions carry pinnules. 

 There are three or five interradials in each of the interradial spaces, and 

 there may be one large, or three small anal plates. The column is rounded 

 near the calyx, but its lower joints are oval and compressed. There is, 

 typically, a large anal proboscis. In connection with the proboscis of 

 Platycrimts, we may just notice the well-known fact that in many speci- 

 mens (as is the case with other Crinoids possessing a similar elongated 

 anal tube) there is found in close apposition with the proboscis, and often 

 placed upon its actual summit, the shell of a fossil Univalve (apparently 

 almost always, or always, a species of Platyceras). It was originally 

 supposed that the Crinoid had been fossilised in the act of eating the 

 Mollusc — the anal tube being regarded as the mouth — but all the living 

 Crinoids feed upon microscopic animalcules, and this supposition is 

 therefore, prima facie, an improbable one. It has also been shown by 

 Meek and Worthen that the Platyceras must have lived for a long time 

 attached to the proboscis of the Crinoid, since the lip of its shell has 

 closely adapted its form to that of the surface to which it is attached. 

 We may therefore safely accept the conclusion reached by these 

 observers, that the Platyceras was in the habit of attaching itself para- 

 sitically to the side or summit of the proboscis of Platycrimts and other 

 Crinoids, thus obtaining a share of the minute animalcules upon which 

 its host lived. 



The genus Platycrimts, as defined by Wachsmuth and Springer, is 

 almost entirely confined to the Carboniferous period, being represented 

 by many species in the Carboniferous Limestone. One or two small 

 forms, however, occur in the Devonian. The genus Hexacrinns is 

 closely allied to Platycrimts, but it possesses a large anal plate, which 

 rests directly upon the basals (as in Actinocrinns). The genus is strictly 

 Devonian, and the species are mostly European. 



By Wachsmuth and Springer the genus Hexacrimts is regarded, prob- 

 ably correctly, as the type of the distinct family of the Hexacrinidce, to 

 which they also refer Hystricrinus and Dichocrimis. Of the remaining 

 genera of the Platycri?iid<£, the only one which needs special mention is 

 the somewhat aberrant Ordovician and Devonian genus Coccocrinus. In 

 this genus, the vault is entirely formed by five large oral plates, which 

 rest upon the interradials and conceal the mouth. 



Family 4. Rhodocrinidce. — The forms included in this family have 

 a dicyclic calyx (fig. 292), but the underbasals may be very small, 

 and are sometimes completely concealed from external view (Glyfito- 

 crinus). The underbasals are usually five in number, sometimes 

 three : and there are five basals. Interradials are well developed, 

 but the anal interradius is "scarcely distinct" (Wachsmuth and 

 Springer). A very characteristic feature, however, is that the first 

 radials are separated by interradials which rest upon the basals. 

 The arms may be uniserial or biserial. The species of this family 

 range from the Ordovician to the Carboniferous inclusive. 



The type-genus of this family is Rhodocrimis itself, which ranges from the 

 Silurian to the Carboniferous. The underbasals in this genus (fig. 292) 

 are five in number, and are well developed. The five lowest interradials 



