30 BULLETIN 88^ UNITED STATES NATIONAL MUSEUM. 



they were suddenly entrapped by the engulfing sand. In the 

 crinid bed of the Keokuk formation at Crawfordsville, Indiana, the 

 ophiurid Ony chaster flexilis is also often met with. 



Aulurids, although as a rule not weU preserved, are probably more 

 often seen than starfishes, and certainly are far more abundant than 

 ophiurids. The latter condition is, however, probably explained by 

 the fact that no ophiurid is known older than the Carboniferous. 

 Of the aulurid Txniaster elegans 31 individuals occur on one small 

 slab. It is probable that if aulurids and asterids were systematically 

 dug for, a far greater harvest would result than that now at hand. 

 Whenever such specimens are found in place, such prospects should 

 be foUowed up by digging or by prolonged search on the part of the 

 local workers. 



It is very seldom that a Paleozoic asterid is so well preserved that 

 all of its parts can be made out, and this is especially true as regards 

 the dorsal side. During the decay of the animals it is probably 

 true that in most cases the individual hes with its ventral side down, 

 and as the soft parts vanish the dorsal skeleton sinks in irregularly 

 over the actinal skeleton. It is the central region of the disk that 

 suffers dismemberment most. Then, as the two sides of the skeleton 

 finally come to be fully compressed, the rays take on an unnaturally 

 great width, so that in many of the fossils preserved in shales the 

 ambulacral furrow is considerably wider than in nature. For 

 these reasons it is often impossible to say whether the ambulacraha 

 are of the opposite or the alternate arrangement, and what was the 

 original structure of the disk and the interambulacral areas. 



ASTERID EVOLUTION. 

 General Observations. 



The majority of the starfishes studied by the wi'iter are from 

 the Ordovicic strata of North America. Collectively these show 

 considerable evolution, for at this early time in the history of the 

 earth Phanerozonia, Cryptozonia, and primitive ophiurids ( = Aulu- 

 roidea) are present. Even near the base of the Middle Ordovicic 

 the two former groups appear to be equally common, while the 

 aulurids ai'e as a rule rare and small, though there are occasional 

 large ones. Here the primitive phanerozonian asterids are also small, 

 hardly ever exceeding 15 mm. in diameter, while the derived and 

 far more complex Cryptozonia are large, some of them having rays 

 exceeding 50 mm. In the Lower Ordovicic of America no starfishes 

 are known, wliile the few that have been reported from Great Britain 

 appear to the writer to be from the Middle Ordovicic. 



These facts show that in spite of their absence in Lower Ordovicic 

 rocks there must have lived at that time various kinds of starfishes. 



