162 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



as "forked plates"), whose superior margins are more or less deeply incised by 

 the radial si7iuses. The term sinus is applied to the open space between the 



two prongs or limbs 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 Nihcleocrinus 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 

 Fig. 252. l^e almost invisible in a side view. Only a part 



Pmtremites godoni (DQh\). Analysis of the deltoids is exposecl to view, their sides 



of calyx, h, Basals ; r, Radials; ir, , . • i i -ii n i • i i 



interradiais or deltoids. being provided With Hanges which are covered 



by the outer ends of the ambulacra. The name 

 dejtoid 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 ambnlacral fields or amlmlacra {" pseudambulacra" of 

 Eoemer), 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- ojJening " 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 



a D 



Fio. 253. 



A, OrhUremites iKirwoodi (O. and S.). Upper face of perfect specimen, with month and anus (a) closed by 

 plates. Spiracles {sp) separate. B, (h-dithocrinua stellljhrmis (O. and S.). Upper face with closed peristome and 

 exposed anus. Spiracles slit-like. (', rcnttvniitett siih-ntus Koem. Central niouth-oiieninj; surrounded by live 

 spiracles, the ]:)ostorior one confluent with the anus. /), Cryftnhldntiis- mclo ((). and S.). Upper face with 

 central mouth-openini;, larj^e anus, and eiL?ht 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 ambnlacral covering plates variously 

 modified in shape. In Nucleocrinus, Orophocrinus and Schizohlasf us sayi, the 

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

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



