CLASS IIT BLASTOIDEA 189 
plates ; with recumbent ambulacra, which support at their lateral margins numerous 
pinnule-like appendages, and are provided along their inner floor with lamellar tubes 
known as hydrospires. 
The calyx is clavate, pyriform, ovate, or globose, frequently pentangular 
at its upper face, and composed of plates which are firmly united among 
themselves. The plates of the abactinal system are arranged in three suc-_ 
cessive cycles, represented by the basals, radials, and interradials. The 
plates of the actinal system comprise the summit plates and the ambulacra. 
The basals consist of two plates of equal size, and a third smaller one, 
which is directed invariably toward the right anterior interradius. Resting 
upon the basals are five V-shaped, usually equal radials (commonly known 
as “forked plates”), whose superior margins 
are more or less deeply incised by the radial 
sinuses. ‘The term sinus is applied to the open 
space between the two prongs or limbs of the Vv} 
plate. 
Succeeding and alternating with the radials, = ce 
and resting upon their limbs, are five interradial iD 
or deltoid plates, which vary excessively in size. Zz yw SS 
In some species they occupy a large part of the 
sides of the calyx, and in others they are confined 
to the upper face. In Llaeacrinus and certain 
species of Granatocrinus, the deltoids extend down 
so far into the calyx as to constitute more than ma aio: 
half, or nearly the whole of its sides, while the — Pentremites Godont, Defr. sp. Dia- 
radials are so short as to be almost invisible in a #° 7 QUcays,  Basale> 7 Radials; - 
side view. Most writers refer the deltoids to the 
plates of the tegmen, but they properly form a part of the dorsal cup. Only 
a part of the deltoids is exposed to view, their sides being provided with 
flanges which are covered by the proximal ends of the ambulacra. The name 
deltoid has reference to the exposed part of the plates, which in most forms is 
triangular or rhomboidal in outline. 
The radial sinuses between the limbs of the radials and the superjacent 
deltoids are filled by the ambulacral fields or ambulacra (“ pseudoambulacra” of 
Roemer). The ambulacra vary in form from petaloid to narrow lanceolate or 
linear, and extend from the summit of the calyx to the distal ends or lips of 
the radial sinuses. The open space in which the ambulacra meet, the so-called 
“ summit-opening,” is pentangular, and central in position. Ordinarily this 
space is unobstructed, but in well-preserved specimens it is covered by a 
greater or lesser number of minute calcareous pieces (Fig. 313); these may be 
either regularly or irregularly arranged, but leave at each angle of the summit- 
opening a small passage-way, by means of which the ambulacra communicate 
with the peristome. The mouth is invariably subtegminal. 
[The summit structure is rarely observed. The covering in Elacacrinus 
consists of five asymmetrical orals ; similar orals, surrounded by smaller plates, 
are present in Orophocrinus, and in Schizoblastus Sayi; but in Granatocrinus 
Billings, E., Notes on the Structure of Crinoidea, Cystidea, and Blastoidea (Am. Journ. Sci. 2nd. 
ser. vols. XLVIII., XLIX., and L.), 1869-70. 
Etheridge, R., and Carpenter, P. H., Catalogue of the Blastoidea in the Geological Department of 
the British Museum, 1886. [Complete bibliography, pp. 303-310.] 
