124 BRITISH PALEOZOIC ASTEROZOA. 



bent upwards in order to capture food. The reasons which inclined me to this view 

 were: (1) The specimens are always found with their apical surface downwards; 



(2) the strong interadambulacral muscles must have bent the arms upwards ; 



(3) the tube-feet were obviously much reduced and coidd have been of little use 

 for walking. Soon after I came to this conclusion, Dr. Gemmill sent me a paper on 

 ciliary nutrition in certain species of Asteroidea (95, pp. 1 — 15) which confirmed 

 me in my view and gave me a new stock of ideas. Gemmill found when investi- 

 gating the actinal ciliation of the Recent pin-cushion starfish, Porania pulvillus 

 (0. F. M.), " with the help of suspended particles, that there were periods during 

 which extremely active ingestion of the particles through the mouth into the 

 stomach occurred." He proceeded to give a list of structural or functional 

 peculiarities which appeared to him to be direct adaptations for ciliary nutrition. 

 There can be no doubt that Gemmill has proved his point, and that Porania can 

 and does live in this way. 



Many of the peculiarities mentioned by Gemmill cannot be tested in the dead 

 Lepidaster, but there are certain characters of Porania which are parallel to those 

 of our Silurian fossil. Thus, Gemmill notes (p. 11) that " the general shape of 

 the starfish with its large flat intermediate [disc] areas, ensures that there is an 

 extensive circumoral ciliated field, adapted for food-gathering purposes." The 

 large disc of Lepidaster has already been noted, and it is perhaps significant that 

 the measurements already given seem to show that the disc becomes relatively 

 larger as the form grows older. 



Again, Gemmill notes (p. 14) that the sucker feet of Porania "are neither 

 particularly strong nor are they kept actively in use." This is exactly as we 

 suspect to be the case in Lepidaster. Gemmill also makes an observation which 

 suggests the use of the long spines of the odontophors of Lepidaster. He states 

 (p. 13) : "If the oral surface of a Porania be sharply irritated, the spines at the 

 interradial angles of the mouth will close in and by interdigitating with each 

 other will cover up the mouth-opening completely. In the same way the whole or 

 any part of an ambulacral groove can be entirely shut in by the spines on opposite 

 sides of the groove. We seem to have here a ready means of protecting the mouth 

 from exposure to streams of inacceptable or injurious particles." 



Lastly, it may be this peculiar mode of life of Lepidaster which has brought 

 about the oral position of the madreporite. It would obviously have been useless 

 functionally if it had been on the apical side pressed against the sea-bottom. 



American Species of Lepidaster. 



No English species are known which appear to be ancestral to any of the 

 Lepidactidae, although the direction of evolution from some form alike in several 

 respects to Mesopalseaster is suggested by the structure of L. wenlocJci. It is 



