vin EGHINODERM ATA MORPHOLOGY OF SKELETON 



367 



a. The apical capsule or dorsal cup. In Platycnnus, the dorsal cup (cf. Fig. 

 254, p. 308) still consists exclusively of the plates of the apical system (three basals 

 and five large radials). The arms are free from their bases. A plate which is found 

 in each interradius, between the bases of the free arms and between the radials, may 

 be considered to belong almost as much to the tegmen as to the dorsal cup. In 

 Hexacrinus the radial structure of the apical capsule is essentially disturbed by the 

 appearance of an anal plate, which presses in between the two posterior interradials 

 in the posterior interradius, and to which, in the direction of the tegmen, two or 

 three other anals may be added. Further, in each radius, the one small primary 

 brachial plate present has become a fixed plate of the apical system. As a further 



FIG. 319. G-ilbertsocrinus tuberculosus, Hall (after Wachsmuth and Springer). The system 

 of plates of the dorsal cup and of the interradial appendages IB. Ba, point of attachment of the 

 anus ; B/, commencement of the free portions of the arms. For other lettering see p. 317. 



example we may take Dimcrocrinus (Glyptasterida;), in which the dorsal cup is still 

 more complicated. In each radius the radial is followed by two primary brachials, 

 which are incorporated into the dorsal cup. In each case the second of these 

 brachials is followed by two or three secondary brachials, which are also fixed in 

 the dorsal cup, the last of them carrying a free arm. In each interradius there are 

 several interradials ; first a large plate which lies between the primary brachials, and 

 then two more lying at the level of the secondary brachials. The posterior interradius 

 is broader than the others. The first plate here lies between the radials, and 

 agrees with them in size, then follows a second row of three plates, and, orally 

 from these, various small plates which lead over on to the tegmen calycis. Inter- 

 distichals may also occur. Melocrinus (Fig. 252, p. 307) and Dorycrinus, etc. agree 

 with Dimerocrinus in these points. 



In Gilbertsocrimts (Rhodocrinidoe) also, the two primary brachials and the two or 

 three secondary brachials are incorporated into the dorsal cup (Fig. 319). In each of 



