CLASS I ECHINOIDEA 275 



amhulacral area. Primordial amhulacral plates arouml the mouth in tlie peristome. 

 Primordial inter amhulacral plates in the basi-coronal row, or usually resorhed. 

 Peristome lüith ten primordial amhulacral^ also non-amhulacral plates, or in one 

 family many rows of amhulacral plates only. Lantern erect or rarely inclined ; 

 teeth grooved, or keeled : foramen magnum deep. Pits in the top of pyramids. 

 Perignathic girdle consisting of low or high apophyses, and auricles. Peristomal gills, 

 rarely with Stewarts organs in addition. Primary spines without a cortical layer. 

 Sphaeridia present. 



Compound amhulacral plates are the most striking feature of this order ; 

 such plates are composed of from two to ten elements, each of which has 

 a pore-pair. The young of the Centrechinoida present stages in development 

 which closely resemble the Cidaroida and also Bothriocidaris. The Centre- 

 chinoida are divisible into three suborders on the basis of the structure of the 

 lantern, which is in brief, teeth grooved, epiphyses narrow, Aulodonta ; teeth 

 keeled, epiphyses narrow, Stirodonta ; teeth keeled, epiphyses wide, meeting in 

 suture over the foramen magnum, Camarodonta. Trias to Recent. 



Suborder A. AULODONTA Jackson. 



Teeth grooved. Epiphyses narrow, not meeting in suture over the foramen 

 magnum. Amhulacral plates simple or Compound. Oculars all exsert, or hecoming 

 insert in the sequence /, V, IV, II, III. Periproct with many plates or granules, 

 or largely leathery. Lantern erect or inclined. Primary tubercles usually perforate. 

 Trias to Recent. 



Family 1. Hemicidaridae Wright. 



Amhulacral plates Compound ventrally, simple above the mid-zone, or in sonie 

 genera Compound throughout. Coronal plates thick, not imbricating. Base of 

 Corona resorhed, Oculars all exsert, or one, or two may he insert. Periproct 

 unknown. Peristome unknown. Lantern erect. Trias 

 to Tertiary. 



Hemicidaris Agassiz (Fig. 385). Amh narrow ; 

 plates near the apical system very numerous, small, 

 low primaries, succeeded by plates formed of from 

 two to four components, together with additional 

 primary or demi-plates. Tubercles in two vertical 

 rows, perforate and crenulate. lAmh broad, with 

 two vertical rows of tubercles similar to those of p^^. gg^ 



t\vQ Amh, but much larger. Oculars all exsert or hemicidaris crenuiaris (Lam.). 



I, or I, V insert. Peristome large, with well- Coral Rag; ChätelCensoire, Tonne. 



developed branchial incisions. Trias to Cretaceous. ^ ^' 

 The following subgenera are recognised : — 



(a) Hemidiadema Ag. Amh tubercles large, and few in number below the ambitus, 

 alternating distinetlj. Jura and Cretaceous. H. stramonium Ag. 



{h) Hypodiadema Desor. Amh narrow, straight ; their tubercles of nearly the same size 

 througliout. Peristome and branchial incisions small. Trias to Cretaceous. 



(c) Pscudocidaris Etall. Amh very undulating abactinally, with primary tubercles near 

 the peristome, granules elsewhere. Jura and Cretaceous. 



Acrocidaris Ag. (Fig. 386). Test large, spheroidal dorsally, flat actinally. 



