229 
III. Thenarocrinus callipygus. 
stichous 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 b 
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 Angulosi , which is the common type in the 
Fistulata. The 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 6, 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 u callipygus^ f 
as an appropriate specific name. 
* “ Ueber den muthmasslichen Geschlechtsapparat von Poteriocrinus 
multiplex , Trd.,” Festschrift k. Gesell. Naturforscher, Moscow, 1882. 
t KaWinvyosj an epithet of a statue of Venus, derived from koXXos , 
beautiful, and nvyr], the posteriors. 
Ann. & Mag . N. Hist . Ser. 6. Vol . vi. 
17 
