502 ECHINODERMATA OPHIUROIDEA 



angles, no less than five or six vertebrae are thus affected, instead 

 of only two as in modern forms. The actual "jaw," however, 

 seems, as in modern forms, to consist only of the first adambu- 

 lacral fused to the second ambulacral, so that instead of conclud- 

 ing with Jaekel that the " jaws " of modern forms result from 

 the fusion of five or six vertebrae, a conclusion which would 

 require that a number of tentacles had disappeared, we may 

 suppose that the gaping " angles " of these old forms have, so to 

 speak, healed up, except at their innermost portions. 



In Boliemura, which belongs to a somewhat younger stratum, 

 the structure is much the same, but the groove in the ambulacral 

 ossicle for the tentacle has become converted into a canal, and the 

 ambulacral groove itself has begun to be closed at the tip of the 

 arm by the meeting of the adambulacrals. 



In Sympterura, a Devonian form described by Bather, 1 the 

 two ambulacral plates of each pair have thoroughly coalesced to 

 form a vertebra, but there is still an open ventral groove, and no 

 ventral plates. 



In the Trias occurs the remarkable form Aspidura, which had 

 short triangular arms, in which the tentacle pores were enormous 

 and the ventral plates very small. The radial plates formed a 

 continuous ring round the edge of the disc. Geocoma from the 

 Jurassic is a still more typical Ophiuroid ; it has long whip-like 

 arms, and the dorsal skeleton of the disc is made of fifteen plates, 

 ten radials, and five interradials. In the Jurassic the living 

 genus Opliioglyplia appears. 



The Cladophiurae are represented already in the Upper 

 Silurian by Uudadia, in which, however, the arms branch not 

 dichotomously, as they do in modern forms, but monopodially. 

 There is a large single madreporite. 



Onychaster, with unbranched arms, which occurs in the Carbo- 

 niferous, is a representative of the Streptophiurae. 



It will therefore be seen that the evolution of Ophiuroidea 

 must have begun in the Lower Silurian epoch. The Strepto- 

 phiurae are a few slightly modified survivors of the first 

 Ophiuroids. By the time the Devonian period had commenced, 

 the division of the group into Zygophiurae and Cladophiurae 

 had been accomplished. 



1 Gcol. Magazine, No. 490, April 1905, pp. 161-168. 



