MORPHOLOGICAL PART. 119 



mouth in the older Crinoids is not accomplished by any additional set of 

 plates, but by the plates which in recent Crinoids surround the mouth. 



Cases in which the ambulacra enter the tegmen from beneath what 

 were formerly called the interradial dome plates, are found in almost 

 every family of the Camerata. They occur more frequently among genera 

 in which the ventral surface is paved by small irregular pieces, such as 

 Gl?/jptocri7vus, Beteocrinus and Archceocrinus ; but also in others. A most 

 instructive case of this kind is presented by a rather young specimen of 

 Megistocrinus nobilis (Plate XL VII. Figs, '^a^ h)^ in which not only covering 

 pieces, but well defined side pieces enter the calyx. The ventral pavement 

 consists of moderately large, irregularly arranged plates, which gradually 

 decrease in size toward the arms. The tegmen is perfectly flat, except near 

 its outer margin, where it is distinctly plicated to form the large openings 

 for the ponderous arms. At the inner flat portions the ambulacra are con- 

 cealed, but at the plicated outer part both covering- and side-pieces come to 

 the surface, and are visible for some distance. It is now quite instructive 

 that in another more adult specimen of the same species (Plate XLYII. Fig. 

 6) the parts of the ambulacra which in the former specimen were exposed, 

 are roofed over from both sides by interambulacral plates of subsequent 

 growth. This observation throws important light upon the development of 

 the so-called vault of the Camerata generally. It shows that the same system 

 of plates, which in a young specimen is in part m^er-ambulacral only, may 

 gradually become s^^j9ra-ambulacral in another. 



We find a somewhat different structure in a finely preserved adult speci- 

 men 0^ Megistocrinus Evansi (Plate XLYII. Fig. 1^), in which in three of its 

 rays two series of large, nodose, alternating plates pass out from near the 

 orals in the direction of the ambulacra. The series are frequently inter- 

 rupted by small, fiat pieces, which are interspersed among the larger 

 ones. In some places the arrangement of the larger plates, which are 

 evidently covering pieces, is as regular as in any Platycrinus ; but at 

 others, owing to the interference of the smaller plates, quite irregular, 

 especially in the two rays to the right of the anus, where scarcely any 

 two of those plates are continuous. It is most remarkable that in no 

 two specimens of this species is the arrangement of the covering pieces 

 alike. In some of them, only the five large bifurcating plates, the so-called 

 radial-dome pieces, are in view, followed by ten others of a second oi^der. 

 The ventral structure of this species not only offers a good proof that the 



