III. Thenarocrinus calUpygus. 229 



sticlious series following would thus correspond to the palmars 

 of an arm. Hitherto the plates have been smooth externally ; 

 they now develop ornament : at the same time, by lessening 

 in height and by gradually coming into a line with one 

 another, they exchange their hexagonal shape for a trans- 

 versely elongate quadrangle. The ornament is produced by 

 the folding of the side-edges of each plate, while the middle 

 remains unaltered, or is raised into a slight hump : the folds 

 of one plate meet those of the plates on its right and left so 

 exactly that in undisturbed parts it is very hard to see the 

 sutures. 



The upper part of the sac, in which the foregoing structure 

 is more developed, may be best studied in specimens 57478 h 

 and 138 Mason College. The ventral sac is nearly as long 

 as the arms, very wide in its lower part, but contracting above. 

 It belongs to the type described by Prof. H. Trautschold * 

 under the head Angulost, which is the common type in the 

 Fistulata. Tlie raised middles of the broad plates, lying one 

 above the other, form longitudinal ridges, which, as in Scaph- 

 iocrinus multiplex^ Trd., sp., appear to be eight in number. 

 That three of these ridges arise by dichotomy from the brachi- 

 anal series is certain ; that another does is probable ; that the 

 others do is possible but uncertain. The depressed tracts be- 

 tween the ridges are occupied by the transversely folded portions 

 of the plates. The anticlinal folds resemble fingers stretching 

 out from the middle of the plate to meet fingers from an adjoin- 

 ing plate ; there may be one, two, or three of these fingers on 

 either side of each plate, but the higher numbers are chiefly 

 found in the proximal and distal regions of the sac (Diagrams 

 2 & 3). The synclinal folds appear as grooves, which are filled 

 with matrix : hence they look like transverse slits proceeding 

 on either side from the suture-line ; but wherever the matrix 

 can be cleared away — a task demanding time and trouble — 

 the floor of the groove is seen to be formed by the solid plate. 

 A natural section, produced by fracture along the suture-line, 

 in 57478 J, likewise shows that all the appearances are pro- 

 duced by simple folding of the plates (Diagram 4). Nor 

 can pores of any other kind be detected. In general appear- 

 ance the ventral sac remarkably resembles a wickerwork 

 basket : the beauty of its structure, its large size, and the 

 extreme development of the anal plates suggest ^^callipygas^'' f- 

 as an appropriate specific name. 



* " Ueber den mutbmasslichen Gesclilecbtsappaiat von Poteriocrinus 

 multiplex, Trd.," Festschrift k. Gesell. Naturforscher, Moscow, 1882. 



t KoKkLTTvyos, an epithet of a statue of Venus, derived from xaXXor, 

 beautiful, aud nvyri, the posteriors. 



Ann. dt Mag. N. Hist. Ser. 6. Vol vi. 17 



