48 The Ottawa Naturalist. [June-July 



sea floor such opportunities became more numerous and the 

 covering pieces began to develop along new lines. 



Let us suppose that we have an original circlet of ten en- 

 larged peristomial epineurals, that these occasionally capture 

 and crush small organisms and that they can be drawn inward 

 by adductors (turning on their long axis a power possessed by 

 all the epineurals of our specimen) and thus carry such particles 

 to, and press them into, the oral cavity. The second pairs of 

 epineurals could also occasionally capture organisms and thrust 

 them under the peristomial circlet or move orad over this circlet 

 when it was in the indrawn position. The first circlet might 

 thus come to function as secondary jaws, moving on their in- 

 turned edges over the sloping oral surface of the adambulacral 

 jaws and developing permanent sliding joints. At a later stage 

 the second epineurals would come to be placed permanently 

 over them and assume the original functions of the first circlet. 



Now, in our specimen we have throughout the food groove 

 an epineural for every adambulacral save the first. Orad of 

 the first, however, and resting on it by a marginal face is a single 

 plate which we must consider as a modified epineural of an 

 earlier circlet which has wholly lost its original function. 



How profoundly this earlier circlet has been modified may 

 be seen by noting the present form of these plates. The mar- 

 ginal faces in contact with the adambulacrals have been widened 

 and beveled to make a good sliding joint fitting the V shaped 

 groove formed by the contact of the latter. This may be seen 

 in plate III, fig. 2, a side view of a pair of these plates and taken 

 before they had been more fully freed from the matrix. The 

 faces apparently resting against the "torus?" are also widened. 

 The outer marginal faces are narrower and consist of an aborad 

 short portion and a longer orad portion that appears to be of 

 the nature of a rounded, blunt, movable spine. The remaining 

 marginal face of each plate shows an inner heavy blunt tooth 

 below the smaller rounded tip of the spine-like piece. The 

 broad contiguous face of each pair was flat and close fitting. 



As the plates of the secondary jaws assumed more and more 

 an indrawn position the second pairs of epineurals moved 

 permanently orad and met over them. The secondary jaws 

 being powerful organs of defense, a complete covering of the 

 peristomial cavity by the second circlet was not necessary. 

 This new circlet (marked as first epineurals in our figures) was 

 thus free to increase the diameter of the central capturing ring, 

 which they did by shifting their attached ends farther aborad. 

 We find that they have encroached on the higher oral face of the 

 interradial marginals and secured thereon well marked excava- 

 tions with a clearly defined semicircular aborad border (plate 



