496 NORTH AMERICAN INDEX FOSSILS. 



/^■\. Arm compartments not present 7*. 



7*. Dorsal cup perfectly symmetrical ; no anal plates present. 

 LXVI. Dolatocrinus. 



7*. Dorsal cup with one or more anal plates </^ . 



5^^ Basals five W. 



b''-'. Arms uni serial LXI. Glyptocrinus. 



W^. Arms biserial , LXIT. Periglyptocrinus. 



5^^. Basals four LXV. Melocrinus. 



^'^ . Basals three LXIV. Macrostylocrinus. 



dd. Radials in contact except at the posterior side where they are 



separated by an anal plate Gf. 



5f . P'irst anal plate heptagonal, followed by a second between 



two in terbrachials 8*. 



8*. Ventral disk highly differentiated, with large and heavy 

 plates, forming a rigid integument. Arms not branch- 

 ing beyond the calyx 6^''. 



6''^. Anus at end of a tube z^^. 



d^ . Interbrachials separated from interambulacrals by 



an arch of brachials 2.'^' . 



1^^^. Calyx biturbinate W\ 



h'^^ . Arms short, equidistant. Anal tube very 



long and central XLVI . Batocrinus. 



W^^. Arms long, paddle-shaped. Anal tube 



excentric XL. Eretmocrinus. 



2.'^^ . Calyx conical. Dorsal cup almost flat. Ven- 

 tral disk very high ; anal tube central. 



XLV 1 1. Alloprosalocrinus. 



c^^. Interbrachials connected with interambulacrals. 



Arms in groups with openings directed upward. 



Anal tube large, central 3^^''. 



' y^^. Arms 20 L. Lobocrinus. 



y^^. Arms l2-l6 LI. Macrocrinus. 



z'', Interbrachials in contact with interambulacrals at 



anal side only , ^^^'. 



\^" . Calyx wheel-shaped. Anal tube very large, 



central. Arms short.. XLVI II. Eutrochocrinus. 



4''''^. Calyx rotund. Anal tube moderately small, 



about central. Arms long..XLIX. Dizygocrinus . 



6''^. Anus without a tube d''''. 



d^^ Calyx lobed y. 



y^^. Arms one from each opening..LI I. Aorocrinus. 

 y^. Arms paired ; spines present on arms and 



tegmen XLl. Dorycrinus. 



d''^. Calyx hemispherical or pyramidal. 



XLII. Agaricocrinus. 



8*. Ventral disk composed of small, irregularly arranged 



plates. Armsgenerally branching beyond the calyx. ..7^^. 



1''. Calyx elongate, urn-shaped. ..XLIII. Periechocrinus. 



7^^. Calyx depressed globose XLIV. Megistocrinus. 



Y^. Calyx low, strongly lobed at arm regions. 



XLV. Gennceocrinus. 



