CLASS nr 



BLASTOIDEA 189 



plates ; with recumbent ambulacra, which suppoii at tlieir lateral margins numerous 

 /linn iila-like appendages, and are provided along their inner floor with lamellar tubes 

 1: a n im as hydrospires. 



The calyx is da vat r, 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 

 plate. 



Succeeding and alternating with the radials, 

 and resting upon their limbs, are five interradial 

 or deltoiil /ilnfi's, which vary excessively in size. 

 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 Elaeacrinus and certain 

 species of Granatocrinus, the deltoids extend down 

 so far into the calyx as to constitute more than FlG 312 



half, or nearly the whole of its sides, while the Pentremttes Godoni, Defr. sp. Dia- 

 radials are so short as to be almost invisible in a fJ^iJj^SiSs mS&SSi ' ' 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 

 t'he 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 Elaeacrinus 

 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. 

 Ether idge, R., and Carpenter, P. H., Catalogue of the Blastoidea in the Geological Department of 



the British Museum, 1886. [Complete bibliography, pp. 303-310.] 



