112 



THE ECHINOIDEA 



Fio. XXX. 



Radial muscle of Asthenosoma. 

 (After P. ami F. Sarasin.) 



the simplest forms of the Echinina both characters are further developed. In 

 the Chalk genera, GlypJwcyphus and Zeuglopleurus (Fig. XXXI.), the sutures 



are excavated by deep hollows, while the plates 

 are thickened by granules and ridges. From 

 such forms as these there is a gradual transi- 

 tion to others with deep pits, which dowel 

 into the plates, as in Temnopleurus (Fig. 

 XXXII.). The transition from the diademine 

 to echinine type of ambulacral plates is 

 shown by Zeuglopleurus and Ortholophus. In 

 the former the plates consist of three fused 

 primaries, separated by free primaries. The 

 middle primary is often very small, and in 

 Ortholophus is often reduced to a demi-plate. 

 The plates then have the arrangement typical 

 of Echinus. From this oligoporous plate 

 the polyporous strongylocentrotoid type is 

 produced by the development of one or 

 more demi-plates between the aboral and 

 adoral primaries. 

 This sub-order began in the Cretaceous. 



FAMILY 1. TEMNOPLEURIDAE. Echinina in which the compound 

 ambulacral plates are formed of three constituents. In the oldest and 

 most primitive forms the three plates are all primaries ; in the later and 

 more specialised types the middle plate is crowded into a demi-plate 

 (i.e. the plates are on the Echinoid type). There is a great development 

 of superficial ornamentation, and the plates are 

 hollowed or undermined by depressions or pits. 

 SUB -FAMILY 1. GLYPHOCYPHIXAE. Temnopleuridae 

 in which the compound ambulacral plates are com- 

 posed of three primaries ; the plates are united by 

 dowelling, but there are no sutural pits. Genera 

 Glyphocyphus, Haime (syn. Rhabdoplcurus, Cott.) ; 

 Zeuglopleurus, Greg. ; Echinocyphus, Cott. ; Paradox- 

 echinus, Laube ; Leiocyphus, Cott. SOB -FAMILY 2. 

 OBTHOLOPHINAE. Temnopleuridae in which the com- 

 pound ambulacral plates are composed of two primaries 

 and an intermediate demi-plate. The plates are united 

 by dowelling, but there are no sutural pits. Genera 

 Ortholophus, Dune. ; Coptophyma, Peron & Gauth. ; 

 Lepidopleurus, Dune. & Slad. ; (?) Triyonocidaris, A. 

 Ag. ; Dictyopleurus, Dune. & Slad. ; Arachniopleurus, 

 Dune. & Slad. (Radiocyphus, Cott.). SUB-FAMILY 3. 

 TEMNOPLEURINAE. Temnopleuridae in which the 

 compound ambulacral plates are composed of two 

 primaries and an intermediate demi-plate. True 

 sutural pits occur, and these often undermine the 

 plates (Fig. XXXII.). Genera Temnopleurus, Ag. ; Temnechinus, 

 Forbes ; Opecliinus, Desor ; Pleurecliinus, Ag. ; Salmacis, Ag. ; Salma- 



XXXI. 



Zeugloplcnrns costulu- 

 tus, Greg. ; plates of 

 upper part of an ambu- 

 lacrum showing simple 

 and compound plates 

 and grooved sutures. 



