ECHINODERMATA. 19 



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

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

 those of the ambulacra; the tubercles are perforated or imperforated, crenulated or 

 uncrenulated, in diflFerent genera ; the apical disc opposite the mouth is small, and com- 

 posed of five genital and five ocular plates ; the poriferous zones are narrow ; the pores 

 are unigeminal or bigeminal. The mouth opening is large and decagonal ; the peristome 

 is divided into ten lobes by deep notches; the spines are long, cylindrical, more or less 

 slender, and are either tubular or solid ; sometimes they are encircled by spiral verticillate 

 processes, or their surface is sculptured with fine longitudinal lines. 



Types. Astropyga radiata, Leske. Diadema pseudo-diadema, Lamarck (PI. VIII, 

 fig. 1). Cypliosoma Konigii, Mantell. Hemipedina Marchamensis, Wright (PI. XI, fig. 1). 



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

 tubercles, small and 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. Mouth opening large, inferior, 

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

 Anal opening small, superior, opposite the mouth, surrounded by five genital and five 

 ocular plates ; buccal membrane naked. 



Types. Arbacia Forbesii, Wright (PI. XIII). GJypticus liter oglypJdcus, Miinster 

 (PI. XIII). Echinus higranularis, Lamarck (PI. XIV). 



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

 primary tubercles, which are 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. Anal opening superior, surrounded 

 by the plates of a large apical disc, composed of more than ten pieces, and which occupies 

 a wide aperture in the superior part of the test. Poriferous zones narrow, pores unige- 

 minal, 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 /lemicidaroides, Wright (PL XVI). 

 GoniopJiorus lunulatvs, Agassiz. 



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

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

 the plates of both covered with numerous small, perforated, and crenulated tubercles ; 

 poriferous zones narrow, pores unigeminal, except near the base, where they are tri- 

 geminal. Mouth opening inferior, central, circular, or pentagonal, armed with five jaws ; 



