THE CRINOID HYPOCRINUS. 



909 



The projection of this facet is due not merely to the protu- 

 berance on the radial, but also to the greater height of that 

 radial and of the subjacent posterior basal. The sum of these 

 heights is again relatively exaggerated by the nearer approach of 

 the left posterior side of the theca to the vertical as compared 

 with the right anterior side, which bulges outwai'd. Thus the 

 oral surface of the theca slopes downwards from the left posterior 

 perradius towards the right anterior interradius. If the erect 

 position of the stem be admitted, then all these departures of the 

 theca from the normal symmetiy result in raising the supposed 

 large 1. post, arm further and further from the sea-floor. 



The evidence thus far does not absolutely warrant the con- 

 clusion that the r. ant., ant., and 1. ant. arms had become atrophied 

 out of existence, or even that they were so reduced as to be use- 

 less as food -collectors ; but it does lead us to infer that the task 

 of sending a food -stream to the mouth was mainly, if not entirely, 

 thrown on the 1. post. arm. 



The f ollowinsr are measurements of the radials in millimetres: — 



Peristome (PI. XO. tig. 9). — The measurement " width above" 

 indicates how much of each radial enters into the peristomial 

 margin. The opening is " elliptische," as Rothpletz describes 

 it, only because the plates are shifted ; probably it was of 

 irregular outline, with an angle about the middle of each of 

 the four large radials, and a longer side in the posterior region. 



One would like to know how this peristome was closed : by 

 oral plates, by other plates of interradial position, by enlarged 

 cover-plates of the left posterior arm, or, as seems highly pi-obable, 

 by the brachials of the reduced arms of the tiivium. 



Periproct (PI. XC. fig. 9).— Dr. Rothpletz regarded the 

 subcircular or subhexagonal opening between the post, and 

 r. post. BB, and the 1. post., r. post., and r. ant. E.R as the 

 periproct. He did not observe all the difficulties to which this 

 interpretation led, such as the apparent absence or almost com- 

 plete atrophy of a radial, or, still more, the unusual position thus 

 assigned to the small infrabasal, or even the necessary conclusion 



Proc. ZooL. See— 1913, No. LXI. 61 



