116 



oro-anal orifice (my, figs. 73 76. The sutures on each side of this plate 

 are generally distinctly visible, especially in the upper part of the body. 



The ambulacra are narrow — one line wide in a specimen fifteen lines in 

 length, with a fine median groove, about large enough to accommodate a 

 tube of the size of a horse-hair. There are two rows of pores, those on 

 one side of the groove alternating in position with those on the other side . 

 These pores lead into the hydrospires. There appear to be only two rows 

 of ambulacral ossicles. The pores are situated in the sutures between 

 them. On each side of the ambulacrum there is a broad transversely 

 grooved marginal plate. From each pore a small rounded ridge runs 

 across this plate. The grooves between the ridges orginate at the outer 

 extremities of the ambulacral ossicles. In well-preserved specimens the 

 surface of these marginal plates exhibits no other structure than the trans- 

 verse grooves and ridges ; but in one weathered specimen that I have examin- 

 ed they seem to be composed of a number of narrow elongated pieces, 

 arranged transversely, in such a manner that two of them abut against 

 the outer extremity of each of the ambulacral ossicles, and extend out- 

 ward toward the interradials. This seems to prove that the marginal plates 

 belong to the ambulacra, as pointed out by Mr. Lyon, and not to the 

 interradials, as represented by other authors. Although I have studied a 

 large number of specimens, none of them were sufficiently perfect to 

 enable me to make out the whole structure of this part of the test of Nu- 

 deocrinus. I have, however, seen enough to convince me that the ambula- 

 cra are much more complex than is usually supposed. The lancet plate, 

 if it occur at all in this genus, must be very narrow. The ambulacral 

 groove, as in Fentremites, sends off branches, right and left. There 

 is also evidence of the existence of minute marginal plates on each side of 

 the groove. 



The hydrospires are ten elongated sacks, each 

 with two deep folds. They are perfectly homolo- 

 ous with those of Pentremites, only differing there- 

 from in not being united in pairs; consequently 

 there are ten spiracles instead of five. The mouth, 

 or oro-anal orifice, is larger in proportion to the size 

 of the body than it is in Pentremites. Mr. Meek 

 through a specimen which informs me that the mouth in some of the Blastoidea 



has all the hydrospires . i i • i i • • 



preserved, h, the two is protected by a smgle valve that covered it like 

 porrSing^^SrfS hy- the lid of a jug. From the structure of the orifice, 

 l^^S'I!' ^' ^^^^ I am inclined to think that in Nudeocrinus it pos- 



grooves. 



sessed a similar protection. 



