GIRVANASTER SCULPTUS. 



71 



supero-marginalia. The remaining radialia are contiguous, and opposite to, rather 

 than alternate with, the supero-marginalia. They are almost oblong, and continue 

 throughout the length of the arm. A very small ocular terminates the series. 



The first pair of supero-marginalia are tall and high, and, as already mentioned, 

 meet across the median axis of the arm. The second supero-marginal has its 

 outside edge in the same straight line as that of the preceding ossicle, but is 

 conspicuously smaller. The third, fourth, and fifth supero-marginalia decrease 

 gradually in size. The remaining five members of the series are very small, and, 

 in consequence, occupy only a small portion of the length of the arm. Some or all 



•.u 



35 



Text-fig. 34 (on left). — Plan of the apical surface of the disc and one arm of Qirvanaster seulptus (based 



on D. 152). X 12. 

 Text-fi«. 35 (on right).— -Plan of the oral surface of the disc and one a^-m of Girvanaster seulptus (based 



onD. 169). X 12. 



of these more distant ossicles are not shown in the majority of the moulds. It is 

 only exceptional good fortune which allowed of their complete preservation. 



Nearly all the supero-marginalia are finger-shaped, and the more proximal 

 ossicles, in particular, are strongly swollen. In consequence, it is only by focussing- 

 somewhat deeply that they are seen to be in reality almost in contact and not 

 separate as shown in the figured plan, in which the space between the neighbouring 

 ossicles is somewhat exaggerated. The swollen ossicles slope so that only a 

 narrow slit is left between them. Some forms do not show this slit. It is obvious 

 that considerable movement was possible. The slits are widest when the arm is 

 high and narrow, and disappear when the arm is flattened and broad. Probably 

 these movements of the arm, and those of the disc already referred to, were 

 brought about by compression of the coelomic fluid. 



Oral Surface (Plate III, fig. 6; Text-fig. 35). — The most conspicuous plates 



