URASTERELLA. 135 



the edges of four infero-marginalia. Text-fig. 87 shows a view of the mouth-parts 

 of another arm of the same individual looked at from inside the mouth (adorally). 

 The first ambulacra! is a very stout plate with a median concavity bordered by 

 two ridges. This same concavity is continued upwards into a hollow in the mouth- 

 angle plate. It serves to house the first tube-foot. It will also be noticed that 

 there are opposite to the first three ambulacralia shown, one mouth-angle plate and 

 two adambulacralia, not one mouth-angle plate and three adambulacralia as is so 

 frequently the case in the Asterozoa. 



Hudson (93) has given a good photograph of the mouth-parts of the American 

 species, U. pulchella (reproduced here, PI. XI, fig. 7). He also kindly sent me a 

 stereogram of the mouth-parts of this species showing a slightly different stage in 

 his laying bare the structures to that he has figured. It shows essentially the 

 same structures as those observed in the English species. Hudson noted (op. cit., 

 p. 123) the large first ambulacralia with their deep pit. He was inclined to regard 

 the pit as serving " for an adductor which assisted in drawing inward the first or 

 interradial pair of cover-plates " (p. 122). In a private letter to me, which enclosed 

 the stereogram, he asks me to note " the paired muscle-remains (one of which is 

 nearly lost) just within the large interradial [odontophor]." He goes on to state 

 that there must have been muscles to draw in the mouth-angle plates, which in 

 each interradius acted as an outer jaw. The origin of these adductors may have 

 been on the inner surface of the odontophors rather than on the first ambulacralia. 

 I see no reason for doubting these later conclusions of Hudson. As already has 

 been pointed out, the pit on the first ambulacral is not a muscle-articulation but a 

 depression for a tube-foot. On the other hand, the stout odontophor may well 

 have served as a fulcrum for muscles moving the mouth-angle plates. 



None of the English specimens show an apical view of the mouth-parts. 

 Hudson, however, has obtained a good apical view of the mouth-parts of U. medusa. 

 I have reproduced one of his photographs here (PL VIII, fig. 4) because it shows 

 such a clear resemblance to the corresponding structure in Cnemidactis (PI. VIII, 

 fig. 5). One of the interesting features in Cnemidactis is the high odontophor, 

 visible in the photograph as a rounded grooved plate immediately above the 

 mouth-angle plates. The photograph of U. medusa shows that the odontophor is 

 similar and equally important in the Urasterellidse. One would not guess this, 

 however, because it is vertical and almost always hidden by the apical plates. 

 Schuchert in fact (85, p. 174) regarded the odontophor as being only present in 

 the young forms. It obviously, as a rule, is difficult to see. Careful search shows, 

 however, that it is always present. 



Genus URASTERELLA, McCoy (emend.). 



1855. Urasterella, McCoy, Brit. Palseoz. Foss., p. 59 (not denned). 

 1885. Stenaster, Billings (pars), Canadian Organ. Rem., iii, p. 77. 



