226 ECHINODERMATA ECHINOIDEA SUB-BRANCH m 



small tubercles, placed near the borders of the poriferous zone. I Ami) primary 



FIG. 362. 



Tetracidfiris Reynesi, Cotteau. Neocomian ; Vergans, near Castellane, Basses Alpes. A, Test reduced one 

 half. B, Portion of Amb, enlarged (after Cotteau). 



tubercles very large, crenulate, and perforate. Spines narrow, elongate. 

 Lower Cretaceous (Barremien) ; Europe. 



Order 2. DIADEMATOIDA. Duncan. 



" Euechinoidea with an actinal, central peristome, and an abactinal periproct 

 situated within the dor so-central system; with perfect or rudimentary or absent 

 internal branchiae, with external branchiae and incisions in the peristome ; with jaws 

 and teeth and a continuous perignathic girdle; ambulacral plates alone continued 

 beyond the peristome or as separate buccal plates. Spheridia present." Duncan. 



Sub-Order A. STREPTOSOMATA. Duncan. 



Test more or less flexible, with external and internal branchiae. Ambulacral 

 plates alone continued beyond the peristome to the stoma. 



Family 1. Echinothuridae. Wyville Thomson. 



Test thin, flexible. Coronal plates with imbricating membraneous edges. External 

 branchiae small, internal sometimes very large. Tubercles small; spines short, 

 cylindrical. 



This family is represented by two living and two extinct genera, the latter 

 being known only by fragmentary specimens. Pelanechinus, Keeping, is found 

 in the Upper Jura, and Echinothuria, Woodward, in the Upper Cretaceous of 

 England. Phormosoma, Wyv. Thomson, and Asthenosoma, Grube (Calveria, 

 Wyv. Thomson), inhabit depths greater than 100 fathoms. 



Sub Order B. STEREOSOMATA. Duncan. 



Test rigid, with external branchiae and rudimentary or absent internal branchiae ; 

 with isolated buccal ambulacral 



