570 



NORTH AMERICAN INDEX FOSSILS, 



bulb often elongated, round in the lower part, obtusely quadran- 

 gular above. (Type of genus.) 

 Hamilton of New York. 



Fig. 1908. Ancyrocrinus spinosus, lateral and summit views. (After Hall.) 



Branch ASTEROZOA Leuckhart. 

 Class I. OPHIUROIDEA Gray. 



The Ophiurians, or brittle stars, are marine echinoderms abun- 

 dantly represented in the modern fauna. They appear as early as 

 the Siluric and have representatives throughout the Palaeozoic. A 

 more or less sharply defined central disk 

 contains the mouth and digestive cavity, 

 which latter does not extend into the slen- 

 der, rounded arms. The arms consist of an 

 axis of jointed calcareous disks (vertebral 

 ossicles), surrounded by plates or a leathery 

 integument and destitute of open ambu- 

 lacral grooves. The madreporic body is 

 situated on the oral (actinal) side of the 

 disk. The arms are movable and serve for 



Fig. 1909. Ony chaster 

 fiexilis, specimen with folded 



arms and outer integument locomotion. They are often intricately in- 

 tertwined (Fig. 1909). 



Two orders are recognized: 

 I. EuRYALE^, in which the arms gen- 

 erally divide dichotomously soon after their 

 origin (though in some cases simple through- 

 out (Fig. 1909)), and covered with a granulated or scaly integu- 

 ment; while a madreporite may be present in all the interrays. 

 American examples : Onychaster barrisi Hall, Burlington of Iowa ; 



of disc and dorsal side of in 

 ner end of some of the arms 

 removed, exposing parts sur- 

 rounding mouth. (After 

 Meek and Worthen, 111. 

 Geol., V.) 



