472 FRANK SPRINGER 



branous, reinforced by a close growth of calcareous particles similar 

 to what has been observed in Taxocrinus, Uintacrinus, and the disk 

 of many Recent Crinoids. It extends far out along the rays, bor- 

 dering the ambulacra as far as they have been traced. It is extremely 

 frail in the fossil, and is of a slightly darker color than the calyx 

 plates. 



In this species the anal plates are very prominent, rising from 

 the posterior basal in the form of a finger-like series, which makes 

 a reverse curve toward the ventral side, the distal end g^ing to the 

 right. It is finely preserved in several of the specimens, but does 

 not appear so plainly in Fig. i, Plate IV, where it has fallen under the 

 posterior edge of the large oral; but in Fig. 3 it is perfectly shown. 

 It is composed of strong and solid plates, well rounded dorsally, and 

 connecting at the inner side with the membranous integument of the 

 disk, which is sometimes found folded longitudinally alongside of it. 

 This anal appendage does not seem to be actually a tube, as I cannot 

 detect any longitudinal opening or canal through it. In one speci- 

 men I find it standing apart from the other plates, and the inner 

 side entirely enveloped in the membrane — thus indicating that the 

 row of strong plates more probably served as a support for a soft 

 tube or opening formed by the protuberance of the membrane itself. 



The exact position of the anal opening has not been observed, but 

 it was no doubt at or near the end of the anal series. So far as I 

 can see, the distal end becomes merged in the perisome of the disk, 

 and it is probable that the aperture for the hind gut was at that 

 point. In all the specimens the structures are broken down and 

 displaced in this part, being involved in the downfall of the large 

 posterior oral, so that the exact fact cannot be ascertained. I have 

 no doubt that the anal structure was substantially the same as in 

 Taxocrinus intermedins, in which, however, the anal opening has not 

 been definitely located. 



In considering the significance of the disk of Onychocrinus as 

 disclosed by these discoveries, we must refer to the observed course 

 of development of the larva in one of the living representatives of 

 the Flexibilia, Antedon rosaceus. Of this I take the following abstract 

 from Dr. W. B. Carpenter's account' of this species in its Penta- 



I Philosophi'al Transactions, 1866, p. 727. 



