PRINCIPAL F0BM8 OF SEA-UUCHINS. 123 



Taking a rapid survey of the principal forms of sea- 

 nrchins, we may divide the class of Echinoidea into two or- 

 ders : the Palecliinida, or older sea-urchins, in which the 

 shell is composed of more than twenty rows of plates ; and 

 the Autechinida with twenty rows of plates.* 



07-der 1. Pafoc/itwiJa.— Comprises first the suborder Me- 

 lonitida, in which there are more than ten rows of ambula- 

 cral plates, represented by Melonites of the coal formation, 

 and Protechinus, Palcechinus, Arcliwocidaris, etc. In the 

 second suborder Eovidaria, there are ten rows of ambulacral 

 plates, A type of the group, Eocidaris Kaiserlingii, appears 

 in the Permian formation. 



Order 2. Autechinida. — 

 To this division belong sea- 

 urchins with twenty rows of 

 plates. The first suborder is 

 the Desmosticlid, comprising 

 those sea-urchins with band- 

 like ambulacra extending 

 from the mouth to the oppo- 

 site extremity, and of more 

 or less regular, flattened, 

 spherical form. Such are 

 Cidaris, Ecliinus, Echinom- 

 etra, Clypeaster, and Echi- 

 nara'ilmius. The Echinus 

 esculenhts Linn., of the Mediterranean Sea, is as large as 

 an infant's head, and is used as an article of food. 



In Clypeaster the body is large and the shell very solid. 

 C. suhdepressus Agassiz is common on the Floridan coast. 

 An orbicular flattened type are the sand-cakes, of which the 

 Echinarachnius parma Gray (Fig. 86) is abundant in the 

 shallower portions of the North Atlantic, from low-water 

 mark to forty fathoms. It is replaced southward from 

 Nantucket to Brazil by Mellita testudinata Klein. 

 The last suborder, Petalosticha, is characterized by the 



•\ few 



Pig. 86. — Echinarachnius parma, com- 

 mon Saud-cake. Natural 6ize.— After A. 

 Agassiz. 



* These are terms proposed by Haeckel, who regards these divisions 

 as subclasses, but we think they should more ijroperly be called orders. 



