214 ECHINODERMATA. 



surface of the arms. The mouth is in the center of the disk, and the ramifications 

 of the stomach extend a greater or less distance into the arm-furrows. In the 

 Ophiuroidea [Op/its, snake; oura, tail] there are usually five simple curving or flow- 

 ing arms with undefined furrows and furnished with cirri, which give them a ragged 

 and tangled exterior. 



The class Echinida is composed of animals having a complete exterior calcare- 

 ous shell of closely-fitting plates, which prevents all flexion of the body. The animal 

 has no arms, but the holes, through which the sucking feet are protruded, are 

 arranged upon five rows of plates running from the center of the top of the shell to 

 the angles of the mouth at the bottom ; or, when they are confined to the dorsal sur- 

 face, they form a distinct five-rayed star surrounding the apex of the shell. A 

 striking character in this class is the manner in which spines are articulated upon 

 tubercles on the surface of the shell ; the base of the spines being hollowed for the 

 reception of the convex surface of the tubercle, and, being sustained in place by a 

 ligament, the spines are movable, and serve economical purposes. The Palaeozoic 

 order Perischoechineda is extinct, but some of them had an internal masticatory 

 apparatus that will compare with any that exists in the living representatives. 



CLASS CRINOIDEA. 

 ORDER PAL^OCRINOIDEA. 



FAMILY ACROCRINID^E. Acrocrinus. 



FAMILY ACTINOCRINID^E. Actinocrinus, Agaricocrinus, Alloprosallocrinus, 



Amphoracrinus, Batocrinus, Dorycrinus, Eretmocrinus, Genuseocrinus, Me- 



gistocriuus, Melocrinus, Physetocrinus, Saccocrinus, Siphonocrinus, (?) 



Steganocrinus, Stereocrinus, Strotocrinus, Teleiocrinus. 

 FAMILY AGASSTZOCRINID.E. Agassizocriuus. 

 FAMILY ALLAGECRINID.E. Allagecrinus. 

 FAMILY ANCYROCRINID^E. Ancyrocrinus. 

 FAMILY ARTHRACANTHID^E. Arthracantha. 

 FAMILY BELEMNOCRINID^E. Belemnocrinus. 

 FAMILY CALCEOCRINIDJE. Calceocrinus, Deltacrinus. 

 FAMILY CAMAROCRINID^. Camarocrinus. 

 FAMILY CATILLOCRINID^;. Catillocrinus. 

 FAMILY CUPRESSOCRINHWE. Aspidocrinus. 

 FAMILY CYATHOCRINIDJE. Ampheristocrinus, Arachnocrinus, Atelestocrinus, 



Barycrinus, Carabocrinus, Cyathocrinus, Erisocrinus, Eupachycrinus, 



Euspirocrinus, Menocrinus, Palseocrinus, Vasocrinus. 

 FAMILY DICHOCRINID^E. Cotyledonocrinus, Dichocrinus, Pterotocrinus, Tal- 



arocrinus. 



FAMILY DIMEROCRINID^;. Coronocrinus, Cytocrinus. 

 FAMILY EDRIOCRINFDJE. Edriocrinus. 

 FAMILY EUCALYPTOCRINID^E. Eucalyptocrinus. 

 FAMILY GASTEROCOMID^K. Myrtillocrinus. 

 FAMILY GAUROCRINID^E. Gaurocrinus, Retiocrinus, Rhaphanocriuus, Thysa- 



nocrinus. 

 FAMILY GLYPTASTERIDJE. Glyptaster, Lampterocrinus. 



