210 BRITISH PALEOZOIC ASTEROZOA. 



and so producing respiratory currents, exactly as Cuenot has shown to be the case 

 in the recent Astropecten and Luidia. (These ridges on the infero-marginaha in 

 Astropecten are shown in Text-fig. 141, p. 202.) We appear therefore to be able to 

 trace progressive changes in Schuchertia, associated with a habit of retiring on 

 occasion, if not altogether, beneath the surface of the bottom. It is probable that 

 the animals did not go very deep. The recent Luidia when at rest is so near the 

 surface that the respiratory currents form "star-like impressions agreeing with 

 the starfish in size and form." The fragmentary condition in which 8. laxata is 

 found both in Europe and America (see p. 212) suggests that it could not have been 

 deeply buried at death, but that slight current-action could displace the ossicles 

 when no longer held together by living tissue. 



The cross-section given (Text-fig. 1 45) shows that only infero-marginalia are 

 present. We can interpret this as meaning either that supero-marginalia have been 

 lost, as in the recent Luidia, or that like Urasterella (p. 130) the form has not yet 

 acquired supero-marginalia. Schuchert takes the first view, and regards the 

 general resemblance of the oral surface to that of Hudsonaster as being due to a 

 descent from that Asteroid. 



I do not think Schuchert is correct, for, apart from the possible descent 

 of Sclbiichertia from a form similar to Eoacth, an examination of 8. stellata 

 does not show the markedly defined infero-marginalia characteristic of the later 

 species, but a far greater resemblance to their less distinct appearance in Urasterella. 

 It is the very prominent infero-marginalia, far more apparent in the later than in 

 the earlier species, which make the form so very like Hudsonaster \ and if the 

 above argument be correct, the resemblance arises secondarily and is not a primitive 

 character. Personally I should be inclined to assign the presence of the differen- 

 tiated infero-marginalia to a progressive flattening of the body, which causes a 

 sharp edge to the disc (see p. 202). 



A progressive diminution in the comparative size of the adambulacralia is dealt 

 with on p. 185. 



An exceedingly interesting feature is the constancy in shape of the mouth-angle 

 plates (Text-fig. 143, p. 203) throughout the life-history of the genus. They are 

 long, narrow, and sharply bent- at the junction of the oral and adoral surfaces. We 

 shall see that Palasterina also has very characteristic mouth-angle plates recognis- 

 able through a considerable geological period. 



As the species make so interesting a series, I give a full description whether or 

 no they are known in British rocks. The following table gives their diagnostic 

 characters : 



8. stellata. — Small form with short arms. R equals in larger specimen 

 9 mm. and is approximately twice r. Adambulacralia about as broad 

 as the infero-marginalia. Apical plates closely fitting and slightly 

 tumid. Middle Ordovician. 



