STRUCTURE AND DEVELOPMENT OF THE AMBULACRUM. 141 



of the area ; the central columns of plates also are quite regular in size 

 and form, whereas the laterals are highly irregular, as shown in plate 2, 

 figure 4 ; plate 4, figure 18. The curvature of the surface of the ambu- 

 lacra, which gives the melon-like ribs, is not existent at the ventral area. 

 This and the other characteristic generic features of this area were not 

 developed in that portion of the test which lies near the peristome. 



Besides the four columns, a, a\ b', b, which are found already at the 

 peristomal border, new columns of ambulacral plates are added dorsally 

 as the animal grew. The new columns are added ^ in the two lateral 

 depressed furrows of each ambulacrum, and in each case between the 

 lateral and median columns of the side to which it belongs, as seen in 

 plate 2, figure 4, and plate 4, figure 18. In our specimen, as shown in 

 the figure (plate 2, figure 4), the first new columns added, c and c?, origi- 

 nate at about the same horizon in each half ambulacrum. The fourth 

 columns of each area as added, e and/, again originate at the same point 

 in the two sides of the ambulacrum. The number of columns in the am- 

 bulacra increases rapidly, but the plates of the ambulacral area are so 

 irregular and usually so imperfectly defined that they are difficult to 

 make out, and only a few specimens have been seen in which the devel- 

 opment of this area could be satisfactorily traced. Besides figure 4, the 

 development of the ambulacrum and the introduction of its newly added 

 columns are shown well in areas B, D, and F of figure 2, plate 2. 



A section of the ambulacrum of Melonites multiporus (plate 2, figure 5) 

 shows the relative size, thickness and position of the several columns of 

 ambulacral plates. The specimen is quite normal in form, not being at 

 all distorted. The adambulacral plates / /of adjacent interambulacral 

 areas project under the lateral plates a b of the ambulacrum, not over 

 them, as they do in Lepidocentrus. The median plates a' b^ of the area 

 are very thick, as well as large in other proportions. The pores in the 

 ambulacral plates on the surface or distal side lie in that portion of the 

 plate which is nearest the interambulacrum (figures 4 and 5), but in trav- 

 ersing the thickness of the plate they extend inward or toward the center 

 of the area, as shown in figure 5. In this section plate Xwas cut in such 

 a plane that the pores did not show. Some of the pores in the section 

 are seen passing quite across the plate. When their whole extent is not 

 visible, their probable position is indicated in the figure by dotted lines. 



The position which new columns take as introduced in the ambulacra 

 in relation to the initial columns a a' and b' b is important, especially in 

 connection with the condition seen in OUgoporusaa discussed in the next 

 paper under the consideration of that genus. The four initial columns 

 of Melonites can properly be homologized with the four columns seen in 

 adult Ollgoporus (plate 6, figure 30), and this is important, because the 



