PRINCIPAL FORMS OF SEA-URCHINS. 



Taking a rapid survey of the principal forms of sea- 

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

 ders : the Palechinida, 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.* 



Order 1. Palechinida. Comprises first the suborder Me- 

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

 cra! plates, represented by Melonites of the coal formation, 

 and Protechinus, Pakechinus, Archceocidaris, etc. In the 

 second suborder Eucidaria, 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 Dcsmosticlia, 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, Echinus, Echinom- Y{K U9 __ Echlnarachnius parma ^ com . 



etra Clliveaster, and Edli- monSaud-cake. Natural size. -After A. 

 * . Agassiz. 



nareihnius. The Echinus 



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



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



In Cli/peaster the body is large and the shell very solid. 

 C. suldepressus Agassiz is common on the Floridan coast. 

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

 Ecliinaraclmius parma Gray (Fig. 149) 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 



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

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



