G2 



BRITISH PALEOZOIC ASTEROZOA. 



(A ) Changes Connected with the Increase in Size of the Disc. 



(1) CJiiiiKjcs connected ivith the Oral Surface. 



These can best be followed l)y comparison of the component fignres of Text- 

 fig. 32. In the axil of the arms of the Hudsonasteridse there is a single plate 

 called by Sclinehert the axillary interbrachial. It is really, I believe (see pp. 72, 

 UG), the odontophor exposed to external view, not hidden by the proximal infero- 



Text-fig. 32. — Figures showing the migration of the odontophor and infero-marginalia. /, Coccaster hulhiferux ; 

 IF, Mesopal^aster primus ; III, Promopalceaster beUviiis {aitev Schuchert). The odontophor has probably 

 di'opped away from the siseeimen figured (Schuchert, op. cit., p. 10.5) ; IV, JCenaiter margaritattis (after 

 Schondorf). 



marginalia as in Recent forms. Throughout my descriptions I have, therefore, 

 except in quotations from Schuchert, definitely called this plate the odontophor. 



In the Mesopalgeasteringe the infero-marginalia come together along the inter- 

 radii so as to shut off the odontophor from the margin. In the more advanced 

 forms, the Promopalseasterinse and the Xenasteridae, the disc is often increased 

 considerably in size by the approximation of several of the proximal pairs of 

 infero-marginalia. This method of increase of size of the disc appears to be 

 peculiar to the Palaeozoic Asteroidea. 



