INVERTEBRATES. 173 



In regard to the value and importance of the characters distinguishing these 

 sections, Palaeontologists will probably always differ. Hitherto these differ- 

 ences have scarcely been noticed, even by the most respectable authorities, 

 excepting as one of the means of distinguishing species. From all analogy, 

 however, it seems reasonable to suppose that they were accompanied by corres- 

 ponding modifications in the structure of the softer parts of the animal. It 

 will also be observed that they correspond, in part, almost exactly to the char- 

 acters distinguishing divisions of the allied Actinocrinus group. For instance, 

 the species embraced in the section Pleurocrinus differ from the typical .forms 

 of Platycrinus, almost precisely as Agaricocrinus and Am-plioracrinus do from 

 the typical species of Actinocrinus. 



The genus Platycrinus seems to have been introduced during the Upper 

 Silurian Epoch, though these earlier species generally depart more or less from 

 the typical Carboniferous forms, as we see in the type of the subgenus Cupel- 

 Imocrinus, for instance, and P. plumosus, Hall. The Devonian species hitherto 

 referred to this genus, are said by de Koninck to all belong to the allied genus 

 Hexacrinus. This group certainly attained its maximum development during 

 the Carboniferous Epoch, which it seems not to have survived. 



Subgenus PLEUKOCRINUS, Austin. 

 Platycrinus subspinosus, Hall. 



PI. 15, Fig. G. 



Platycrinus subspinosus, Hall, 1858. Geol. Report Iowa, Vol. I, part II, p. 536, pi. 8, 

 figs. 9 and 10. 



Body, including the summit and second radials, subhemi- 

 spherical, being nearly flat below and gibbous above ; decahedral 

 in outline, as seen from beneath, with the arms and second 

 radials removed. Base pentagonal in outline, flat or scarcely 

 visible in a side view, broadly and deeply concave in the mid- 

 dle; margins distinctly beveled ; sutures a little impressed near 

 the beveled lateral margins. First radials hexagonal, as seen 

 from below, joining nearly horizontally around the base, but 

 curving abruptly upwards on each outer lateral margin to con- 

 nect with the interradial and crown pieces, where they have 

 each an addition lateral angle, not visible from beneath; lateral 



