62 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



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 prorigs or limhs of the plate (Fig. 252). 



Succeeding and alternating with the radials, 

 and resting upon their limbs, are five interradial 

 or deltoid plates, which vary excessively in size ; 

 they are considered to be homologous with the 

 oral plates of the Crinoids. 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 Nudeocrinus and certain species of Orhitremites, 

 the deltoids extend down so far into the calyx as 

 to constitute more than half, or nearly the whole 

 of its sides, while the radials are so short as to 

 be almost invisible in a side view. Only a part 

 of the deltoids is exposed to view, their sides 

 being provided with flanges which are covered 

 by the outer ends of the ambulacra. The name 

 deltoid has reference to the exposed part of the plates, which in most forms 

 is triangulär or rhomboidal in outline. 



The radial sinuses between the limbs of the radials and the super jacent 

 deltoids are filled by the ambulacral fields or ambulacra (^' pseudambulacra " 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 lij)S of 

 the radial sinuses. The open space in which the ambulacra meet, the so-called 

 '* summit- opening " or peristome, is pentangular, and central in position. Ordin- 

 arily this space is open, but in well-preserved specimens it is covered 

 by a greater or less number of minute calcareous pieces (Fig. 253); these 



Fig. 252 

 Pentremites godoni (Defr.J. 

 of calyx. h, Basais ; 

 Interradials or deltoids 



Analysis 

 Radials ; ir, 



Fig. 253. 



A, Orhitremltes norwoodi (O. and S.). Upper face of perfect specimen, with inouth and anus (a) closed by 

 plates. Spiracles {sp) separate. B, Orophöcrinus stelliformis (O. and S.). Upper face with closed peristome and 

 exposed anus. Spiracles slit-like. C, FeMremites sulcatus Roem. Central mouth-opening surrounded by five 

 spiracles, the posterior one confluent with the anus. D, Cryptohlastus melo (O. and S.). Upper face with 

 central mouth-opening, large anus, and eight spiracles (after Carpenter). All specimens from Burlington 

 Group; Iowa. 



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 small plates which cover 

 the peristome are merely extensions of the ambulacral covering plates variously 

 modified in shape. In Nudeocrinus, Orophöcrinus and Schizoblastus sayi, the 

 central Space is occupied by five asymmetrical plates, formerly called orals, 

 surrounded by smaller ones toward the grooves. In Orbitremites norwoodi and 



