CLASSIFICATION. 31 



ambulacral areas wide, with two or four rows of primary tubercles ; inter-atubulacral areas 

 with two, four, six, or more rows of tubercles, nearly of the same size and structure as 

 those of the ambulacra ; tubercles perforated or imperforated, crenulated or uncrenu- 

 lated, in different genera; apical disc small, opposite the mouth, composed of 

 five genital and five ocular plates ; poriferous zones narrow ; pores unigeminal or 

 bigeminal. Mouth-opening large and decagonal ; peristome divided into ten lobes by 

 deep notches ; spines long, cylindrical, more or less slender, either tubular or solid ; 

 sometimes encircled by spiral verticillate processes, or their surface is sculptured with fine 

 longitudinal lines. 



Types. Astropyya radiata, Leske. Cyphosoma Kceniyii, Mantell. Pseudodiadema 

 tumid um, Eorb. 



Family 4. Echinid^e. — Test thin, spheroidal; inter-ambulacral areas, with small 

 primary tubercles, of various sizes, perforate or imperforate ; bosses crenulate or un- 

 crenulate ; ambulacral areas wide, always supporting two or more rows of primary 

 tubercles ; poriferous zones narrow or wide, pores unigeminal, trigeminal, or polygeminal, 

 and disposed in arcs ; spines short, mostly subulate. Month-opening large, inferior, 

 always decagonal ; peristome divided into lobes by notches more or less deep. Vent 

 small, superior, opposite the mouth, surrounded by five genital and five ocular plates ; 

 buccal membrane naked. 



Types. Echinus splicer a, Miiller. Codiopsis Doma, Desmarest. 



Family 5. Salenid.e. — Test thin, spheroidal; inter-ambulacral areas wide, with few 

 primary tubercles, either perforate or imperforate, crenulate or uncrenulate ; ambulacral 

 areas narrow, carrying secondary tubercles. Mouth-opening small or large, inferior, 

 decagonal ; peristome more or less notched. Vent superior, surrounded by the plates of 

 a large apical disc, composed of more than ten pieces, occupying a wide aperture in the 

 superior part of the test. Poriferous zones narrow, pores unigeminal, except near the 

 peristome, where they are trigeminal. Spines long, subulate, circular, or flattened. 

 Species all extinct ; the genera distributed in the Oolitic, Cretaceous, and Tertiary 

 rocks, 



Types. Salenia petalifera, Defrance. Acrosalenia hemicidaroides, Wright. Gonio- 

 phorus lunulatus, Agassiz. Hyposalenia Wrightii, Desor. 



Family 6. EchinocomdjE. — Test thin, circular, elongated or pentagonal, elevated or 

 depressed ; inter-ambulacral areas wide, ambulacral areas narrow ; external surface of 

 the plates covered with numerous small, perforated, and crenulated tubercles ; pori- 

 ferous zones narrow, pores unigeminal, except near the base, where they are trigeminal. 

 Mouth-opening inferior, central, circular, or pentagonal, armed with five jaws ; peristome 



