ECHINODERMATA—ECHINOIDEA. S77 



3. Diadematoida. With continuous perignathic girdle ; ambulacral plates 



alone continued beyond the peristome, or as separate buccal plates ; am- 

 bulacral plates typically with more than one pair of pores e. 



e. Pairs of pores superposed so as to form simple vertical series, or arranged 



in arcs of three XII. Diadema. 



e. Pairs of pores alternating to form biserial columns, at least in part ***. 



***. Biserial pores above ambitus only XIII. Diplopodia. 



^**. Pores in two double rows throughout XIV. Fedinopis. 



e. Ambulacral plates highly compound of primaries and demiplates. 



XV. Cyphosoma, 

 II. Test irregular, with peristome and periproct not at opposite poles 4. 



4, Peristome central, periproct in posterior interambulacrum f. 



f. Holectypoida. A pair of pores or only one pore to an ambulacral plate..4*. 



4"'^. Perignathic processes of ambulacra present ; periproct large between 

 margin and peristome ,XVI. Holectypus. 



f. Clypeastroida . More than a pair of pores to an ambulacral plate 5*. 



5*. Margins rounded and more or less tumid f. 



f . Margin entire J. 



%. High, with base abruptly hollowed to deep central peristome. 



XVII. Diplothecanthus. 



%. Low, base flat, peristome not depressed XXI. Mortonia 



f. Margin notched ; test low and flat XXIII. Encope. 



5*. Margin sharp, depressed, test very flat, with flat base and branched 



ambulacral furrows f|. 



ff. Margins and tests entire, without notches or perforations W. 



W. Apical system central \'^ . 



1^^ . Ambulacral petals extending about half way to margin ; 

 periproct nearer the peristome than margin. 



XVIII. Periarchus. 

 l^^. Ambulacral petals extending to near margin, periproct 



marginal or inframarginal XIX, Sctitella. 



X\. xVpical system excentric, periproct actinal, marginal or supra- 

 marginal XX. Echinarachnius. 



■\\. Margins notched, test frequently with perforations (lunules). 



XXX- 



XXX' 1'^st very flat, with five or six usually closed lunules or 

 notches, one in median posterior interambulacrum. 



XXII. Me Hit a. 

 XXX' Test with broad notch or lunule in each ambulacrum, and 



a lunule in posterior interambulacrum XXIII. Encope. 



4. Spatangoida. Peristome and periproct both excentric, placed along a longi- 

 tudinal axis g. 



g. Ambulacra similar 6*. 



6*. Periproct at upper end of groove on abactinal area of test ; floscelle 



absent or rudimentary XXIV. Echinobrissus. 



6*. Periproct supramarginal, floscelle well developed. .XXV. Cassidulus. 



g. Ambulacra dissimilar 7** 



7*. Ambulacra nearly similar, flush with surface, not forming petaloid 



areas fff. 



ftf . Periproct inframarginal, broader than long, anterior ambulacrum 



not in groove XXVI. Echinocorys. 



37 



