256 



ECHINODERMATA -ASTEROZOA 



PHYLUM IV 



this order, but the form of the ossicles certainly seems to exclude it. 

 Moreover, Eudadia, so far as its structure is known, is capable of very diverse 

 interpretations, and the possibility that it is a Starfish rather than an 

 Ophiuran, must not be wholly ignored. 



Order 4. ZYGOPHIUROIDA. 



Ophiuroidea with simple anns, perfecMy regular series of arm-p>lcites and vertebral 

 ossicles fully developed. The movement of the ossicles on each other is greatly limited 

 by the development of lateral processes and pits on their articulating surfaces. 



This order includes the great bulk of the Recent Ophiurans as well as 



those of the Mesozoic and Tertiary. 

 Indeed some of the Recent genera seem 

 to have been differentiated as far back 

 as the Jurassic, and Ophioderma appar- 

 ently occurs in the Triassic. No 

 Paleozoic forms can certainly be re- 

 ferred to this group. 



The genera Aspidura (Fig. 362) 

 and Acrura Agassiz are occasionally 

 abundant in the German Muschelkalk, 

 and certain Liassic Ophiui'ans were 

 also, like many Recent species, notably 

 gregarious. In the Lower, Middle 

 and Upper Jura are found Brittle Stars 

 closely allied to the Recent Ophiolepis 



B 



Pig. 363. 



Opliiocten kelheimense BiJhm. Lithographic Stone ; 

 Kelheim, Bavaria. A, Ventral aspect of dislj. B, 

 Dorsal surface of one of the arms. (Both figures en- 

 larged ; original in Munich Museum.) 



Fig. 3ti4. 



Geocowi carinnta Goldf. Lithographic Stone ; Zandt, near Solenhofcn, Bavaria. A, Individual of the 

 natural size. B, Dorsal aspect of disk, showing granulations and central depression. C, Ventral surface of one 

 of the arms. (Figs. B and C enlarged.) 



M. and T. ; Ophiocten Ltk. (Fig. 363); Ophiura Lamk. (Figs. 359, 360); and 

 Ophiomusium Lyman. It is possible that some of these are really congeneric 

 with Recent species. The Mesozoic genus Geocoma d'Orb. (Fig. 364) is related 

 to the Recent Amphiura Forbes, but there can be little doubt that some of 

 the species referred to it are based on material which cannot be determined 

 so precisely. Liitken considers Ophiurella elegaiis Ag., from the Lithographic 

 Stone of Solcnhofen, to be a member of the Recent genus Ophiocoma Ag., but 



