240 PHYLUM ECHINODERMATA. 



chiata) and incisions in the peristome margin. With jaws and teeth and 

 continuous perignathic girdle. Ambulacral plates alone continued on 

 to the oral area, where they may appear as separate buccal plates. 

 Sphaeridia, ophicephalous and triphyllous pedicellariae present. Jurassic 

 to the present day. 



Section 1 . STREPTOSOMATA. 



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

 stomial ambulacra! plates in several rings. 



Fam. Eehinothuridae. With the characters of the sub-order. The 

 thin and flexible tests are large and tumid, or depressed. Some of the 

 spines of the interambulacra have poison sacs near their ends and are 

 poisonous. The plates of the apical system are usually separate. Coronal 

 plates feebly calcareous and with membranous edge, with open reticulate 

 structure. Internal longitudinal muscles for moving the plates. A 

 flexible echinoid was described in 1863 by S. P. W'oodward from the 

 chalk. Asthenosoma, the first recent form was described by Grube * in 

 1868, and rediscovered by Wy. Thomson t in the dredgings of H.M.S. 

 Porcupine. The large internal branchiae were discovered by the Sara- 

 sins t in 1888. 



Pelanechinus Keeping, apical plates absent, Oolitic. Echinothuria 

 S. P. Woodward, apical plates absent, Upper Cretaceous ; Phormosoma 

 Wy. Thorns., recent, 120-2,750 fathoms, N. Atlantic to Azores, in most 

 seas; Asthenosoma Gr., recent, 100 to 450 fathoms, N. Altantic and in 

 most seas. Echinosoma, Calveria, Araeosoma, etc. 



Section 2. STEREOSOMATA. 



Test rigid, with external branchiae, and reduced or absent internal 

 branchiae. With 5 pairs of isolated peristomial ambulacra! plates (buccal 

 plates). 



Fam. 1. Saleniidae. Ectobranchiate, with persistent central or 

 centrals. Ambulacra narrow ; the plates are primaries, rarely compound 

 actinally. Interradial plates few, tubercles large. Sphaeridia present. 

 Jaws with the foramina of the pyramids unarched by epiphyses, teeth with 

 a keel. Jurassic to recent. Mostly extinct forms. Peltastes Ag., 

 AcrosaleniaAg., extinct ; Salenia Gray, f and r, Caribbean Sea, etc., 60-1,700 

 fathoms. 



Fam. 2. Hemicidaridae. Exclusively fossil forms, Permian to Cre- 

 taceous. Hemicidaris L. Ag., Acrocidaris L. Ag., Goniopygus L. Ag., 

 genital openings outside the apical system in the interradii ; Circopeltis 

 Pomel, Cidaropsis Cotteau, Glypticus L. Ag. Leptocidaris Quenstedt, 

 allied here. 



Fam. 3. Aspidodiadematidae. With spheroidal test and large, narrow, 

 ringed apical system formed by broad basals and broad intervening radials. 

 Interradial plates few. Ambulacra with low primary plates ; pores in 

 straight series, one pair in each plate. Peristome incised ; branchiae 

 bifid with ten large buccal plates. Tentacles heteropodous. Aspido- 

 diadema A. Ag., 100 to l,700fathoms, Caribbean Sea, N. part of S. Atlantic, 

 Philippine Sea. Dermatodiadema A. Ag. 



* Jahresb. d. Schles. Ges. f. Vaterl. Cult. 1868, p. 42. 



f Phil. Trans. 144, 1874, p. 737. 



j P. and F. Sarasin, Erg. Nat. For. auf Ceylon, 1, 1888, p. 129. 



