GENEEA AND GROUPS OF THE ECHINOIDEA. 127 



Apical system either compact, or some radial plates separating 

 some basal plates ; madreporite in the largest basal. The oval 

 or circular periproct with numerous plates, smallest near the 

 anus. 



Ambulacra narrow to moderately wide, straight ; poriferous 

 zones narrow, pairs in more or less vertical arcs of triplets, the 

 innermost pair the adoral of the three, and in a primary consti- 

 tuent ; the other pairs in demi-plates, or the upper member may 

 be a low primary. Interporiferous areas with two rows of small, 

 plain, primary tubercles, placed vertically, with or without irre- 

 gularly placed secondary tubercles and primaries. 



Interradia with two vertical rows of tubercles resembling 

 those of the ambulacra, or slightly larger, with few or numerous 

 rows of secondary tubercles and miliaries, or a considerable 

 number of small tubercles diminishing in number abactinally ; 

 tubercles most numxerous below the ambitus. 



Peristome rather small, nearly circular ; branchial incisions 

 not large ; peri gnathic girdle well developed, processes tall and 

 arched. Jaws high ; foramen of the pyramid high and arched 

 by epiphyses ; teeth keeled. Buccal membrane with ten buccal 

 plates with tentacles and small spines ; some may have numerous 

 buccal plates with a corrugated aspect. Tentacles homoiopodous, 

 disciferous. Spines either very short relatively or moderately 

 long, striated, sharp, more or less swollen at the base. 



Fossil. Cretaceous to Pliocene and Post-Pliocene: Europe. 

 Pliocene : England. Tertiary : Asia. 



Becent. World-wide, littoral to 2435 fathoms. 



The genus PsammecMnus, Agass., 1846, Catal. Eais., Ann. d. 

 Sci. Nat. vol. vi., and Desor, 1858, Synopsis, p. 118, really differs 

 from the typical Echini in having the buccal membrane with 

 concentric plates besides the ten buccal plates ; but palgeonto- 

 logists have usually termed those species of EcJiinus which have 

 numerous subequal tubercles PsammecMnus. The size of the 

 branchial incisions does not afford a generic distinction. Desor 

 was not warranted in comparing the buccal and otlier plates 

 with those of Cidaris. It is certain that some Echini with few 

 tubercles, as well as others with many, have the peristomial 

 membrane with numerous plates ; and thus the distinction 

 relied upon fails. At the most PsammecMnus is a subgenus of 

 Echinus ; and the sole structural character refers to the peri- 



