THE STEM AM) CALYX. 89 



the anal area as occurs in Qlyptasti r. "The posterior ana is distinctly divided into 



two equal parts by a single median row of plates. This row, which often extends to 

 the anal opening, is composed of the true anal plates, hut the plates on either Bide 

 of it are interradials. By considering the latter, without regard to the median row, 

 it -will be found that the two sections taken together correspond exactlj in number 

 and in general arrangement with the interradials of the Other areas, or at least differ 

 not more than the other four differ among themselves." 



Although the posterior or anal intcrradius of ElceacHnus is the only one which is 

 definitely tripartite, yet there is a distinct tendency, both in this genus and in 

 Schizoblastus Sayi, to an actual division of the interradial area into three longitu- 

 dinal portions. This is of some interest as regards the relation of the Blastoids to 

 the Urchins. For in the Triassic Tiarechinus, which has a remarkable superficial 

 resemblance to an inverted Blastoid, the greater part of each interradial area actually 



Fie. V 



Tiarechinus princtps, Laubc, inverted so as to show the resemblance of its abaetinal system 



to the calyx of a Blastoid. (The figure, but not the letters, after Loven.) 



3. Basal. 4. Radial. /. Single median interradial. i. Lateral interradials. 



does consist of three plates, the two outer ones of which rest partly on the large 

 basal and partly on the small radials, while the median one rests on a basal only 

 (Fig. V.). The coincidence is a curious one, and is worth recording; but we have no 

 desire to be understood as attributing any definite meaning to it, at any rate for the 

 present. 



G. Irregularities of the Calyx, axd Monstrosities. 



In the Blastoids, as in the Crinoids, slight irregularities in the structure of the 

 calyx are not uncommon. A very insignificant one which we have noticed in 

 Cryptoschisma Schulzi is shown in PI. V. fig. 24. The distal end of one of the 

 deltoids shows itself as a small diamond-shaped plate which caps the truncated limbs 

 of the radials on the exterior of the calyx, though nothing of the kind is visible in 



