ECHINOIDEA. 



137 



be protruded and retracted. Tliis description of the Echini will 

 not accurately J^pply to the irregular forms. In the latter the 

 ambulacra are only half of the typical number, and often only 

 partial in tlieir extent. In many, too, the oral and anal open- 

 ings lose their central position, and approach the margin. The 

 mouth, also, is sometimes toothless. The mouth is central and 

 dental in the Cidarida', Echinidce, and Clypeasteridce; it is more 

 or less eccentral and edentulous in the EchinonidoB, Collyritidm 

 and Spatangidiw 



The fossils of this order are usually divided into three groups: 

 CidaridcB or Turban-shaped 'Echini,' Clypeasteridce or Buckler- 

 shaped Echini, and Sjyatangida or Heart-shaped Echini. These 

 include over TO genera and about 700 species. They are rare in 

 the Palaeozoic, — the oldest occurring in the Upper Silurian, — 

 and attained their maximum in the Oolite and Cretaceous strata. 

 The regular forms appeared first ; the elongated forms are more 

 recent. 



No. 413. Ananchytes ovata, Lam. 



A Spatangoid, distinguished by having 

 a high, conical form, and much compressed 

 laterally. The mouth is eccentric, in front; 

 spines minute. This genus, saj^s Desor, is 

 the most characteristic of all fossil Echini, 

 and the one most useful to Geologists, not 

 only from its frequent occurrence, but also 

 from its well defined characters. It is 

 limited to the Mesozoic. This specimen is 

 from the Upper Chalk of Kent, England. 



No. 414. Cidaris coronata, Goldf. 



Spines. In the Cidaris proper the form is 

 symmetrical, the tubercles are perforated, the 

 ambulacra narrow, and the two lines of pores 

 close together. This species is depressed, and its 

 tubercles are few in number and of large size. 

 These specimens are from the Coral Rag (Middle 

 Oolite), Nattheim, Wurtemberg. 



No. 415. Cidaris dorsata, Brown. i. 



Spines. The spines of Cidaris are few in number, large and mostly club- 

 shaped. Trias (St. Caspian), Tyrol, Italy. 



