178 BRITISH PALEOZOIC ASTEROZOA. 



Section E. — Family Eoactinid.e, nova. 



1890. Palteasterinidse, Stiirtz, Palseontographica, vol. xxxvi, p. 240 (contains Palasterina). 



1899. „ (pars), Gregory, Geol. Mag., dec. iv, vol. vi, p. 348 (contains Palasterina, 



Schcenaster, Schuchertia). 

 1899. Lindstromasterinse (pars), Gregory, Geol. Mag., dec. iv, vol. vi, p. 346 (contains Lindstromaster 



and Uranaster). 

 1910. Palasterinidse, Schondorf, Jalirb. nassauisch. Ver. Naturlc, Wiesbaden, vol. lxiii, p. 220. 

 1914. „ (pars) Schnchert, Fossilium Catalogus, Animalia.pt. 3, p. 7 (contains Pet raster, 



Lindstromaster, Palasterina, Uranaster, Palteostella, Pseudopalasterina). 



1914. Schuchertiidse, Scliuchert, Fossilium Catalogus, Aniraalia, pt. 3, p. 8 (contains Schuchertia). 



1915. Palasterinidse (pars), Scliuchert, Bull. 88, U.S. Nat. Mus., p. 138. 

 1915. Schuchertiidse, Schuchert, Bull. 88, U.S. Nat. Mus., p. 194. 



Diagnosis. — Interbrachial areas occupied by downgrowths of the apical 

 interradii separating the bases of the arms. Madreporite oral. (These characters 

 are those distinctive of the Ophiuroidea, see below.) Ambulacral channel open to 

 exterior, ambulacralia not modified into vertebras. (These characters distinguish the 

 Eoactinidge from the recent Ophiuroidea and from the Palaeozoic Ophiuroidea which 

 have been termed Auluroidea.) Adambulacralia usually broad and possessing broad 

 surfaces of contact with their neighbours. When they form the oral margin of 

 the arms they never occupy the whole of the lateral surface — that is, never are side 

 plates. (These characters distinguish the Eoaetini dst 'from other Palxozoie Ophiuroidea 

 which possess an open ambulacral channel and ambulacralia not modified into vertebrae.) 



This section contains a number of forms of distinct Asteroid shape which up to 

 the present have been regarded as Asteroidea, e.g. Eoactis, Schuchertia and 

 Palasterina. They seem to be rather "Asteroid" forms which lie at the base of 

 the Ophiuroidea. 



In order to make this clear it is necessary to examine closely the exact differ- 

 ences between the Asteroidea and the Ophiuroidea. It is usual and natural for 

 authors well acquainted with the living forms, when stating the differences between 

 the two groups, to lay great emphasis upon the adaptations which the Ophiuroidea 

 show for the wriggling habit. Thus MacBride states (43, p. 477) in respect to 

 the Ophiuroidea that " like the Asteroidea, they are ' starfish,' that is to say, they 

 consist of a disc and of arms radiating from it; but the scientific name Ophiuroidea 

 really represents the great dominating feature of their organisation. Literally it 

 signifies ' Snake-tail ' (opie snake ; ovpa tail), and thus vividly describes the 

 wriggling, writhing movements of .the long thin arms, by means of which the 

 Ophiuroid climbs in and out of the crevices between the stones and gravel in which 

 it lives. This feature, viz. the effecting of movement by means of muscular jerks 

 of the arms instead of by the slow protrusion and retraction of the tube-feet, is 

 the key to the understanding of most of the points wherein the Brittle Stars differ 

 from the true starfish." 



