rRASTERELLA THRAIVENSIS. 



130 



surface of the arm is well rounded though somewhat flattened in the median 

 region. Possibly the flattening is due to post-mortem pressure and the arm may 

 have been originally more tumid. The entire oral surface is occupied by the 

 adambulacralia, which also help to form the margin. Succeeding the adambula- 

 cralia are well differentiated infero-marginalia. Between the infero-marginalia and 

 the stout radialia are three rows of radialia. The paxilla-shafts are well seen in 

 this view. 



Measurements. — D. Ill has R : r : : 51 mm. (at least) : 3 mm. Width of arm 

 at base about 4 mm. 



Width of arm at base of D. 254 is 36 mm. 



Width of arm at base of D. 35 is 4 mm. 



Width of arm at base of D. 230 is 3 mm. 



This last specimen seems to show the arm in full length — R : r : : 44 mm. : 

 3 mm. There appear to have been between fifty and sixty adambulacralia down 

 each side of the arm. The infero-marginalia are equal in number to the 

 adambulacralia. 



Text-fio. 80. — Plan of ossicles on the oral surface of UrastereUa thraivensis. O.. odontophor. x 25 



Horizon and Locality. — Upper Ordovician (Ashgillian) of Girvan, Ayrshire, 

 Scotland. 



Food. — A mould of a small gasteropod is just within the mouth-angle plates 

 of D. 254. This is a fact of some interest, as it shows the forms had the power of 

 engulfing fairly large particles, and therefore did not depend upon ciliary action 

 for their food. 



Description of D. 46 and 46r (Text-fig. 81. p. 120). — This small specimen is 

 undoubtedly an immature form of the species. It is much more stellate than the 

 adult ; R : r : : 4 mm. : 1*8 mm. The plates of the disc are exactly as in the 

 holotype. It is interesting to note, however, that the adradialia on the left side 

 of the figure are very incompletely developed. The infero-marginalia are well 

 differentiated. In fact they appear to be more prominent as marginal plates, 

 viewed either orally or apically, than they are in the mature form. Schuchert has 

 also noted this latter fact with regard to the young of U. ulric-hi (85, p. 185). 

 The paxilla-shafts of the radialia are more spicular than in the holotype. I have 

 not been able to distinguish the markings of the madreporite. The transverse 

 ridges of the adambulacralia are not readily made out. 



