STRUCTURE OF THE INTERAMBULACRUM. 147 



Melonites giganteus (plate 5, figure 21), where the fifth column originates 

 in the second row above the fourth. On the other hand, the relatively 

 later period of introduction of column 4 in the two examples of Melonites 

 multiporus seen (plate 3, figures 12 and 16), shows a tendency toward a 

 slower rate of development. In Tiarechinus we have a case of even greater 

 accelerated development, for in it, according to figures by Professor Loven 

 (26), there are three plates in the second row succeeding the initial single 

 plate of the first row. 



One of the clearest examples of the method of plate arrangement seen 

 is a specimen of Melonites multiporits in the collections of the Museum of 

 Comparative Zoology (catalogue number 2990),* Plate 3, figure 12, rep- 

 resents the interambulacral area of this specimen, which is also shown in 

 the photographic reproduction (plate 4, figure 18), and the nature of the 

 plate arrangement may be seen by following up the series of columns of 

 plates which are accentuated by dotted lines. While not complete dor- 

 sally or ventrally, this specimen is very clear and is also very typical, 

 showing little departure from the ideal type of plate arrangement.f 



Starting at the ventral end of the specimen (plate 3, figure 12, and 

 plate 4, figure 18) there are three columns of plates, composed of two 

 adambulacral columns of pentagonal plates, numbers 1 and 2, and a 

 median column of hexagons, number 3. This middle column, number 3, 

 would drop out near the oral termination of the area if it were complete, 

 as in plate 2, figure 2. Passing dorsally, .a new fourth column of plates 

 is added. This column is introduced by the terminal pentagonal plate, 

 number 4. The introduction of this plate has disturbed the mechanical 

 form of adjoining plates, so that they are somewhat distorted, as shown 

 in the figure, and a hexagonal plate exists at A in place of one of the 

 lateral pentagons of column 1. This introduction of an additional side 

 to plate A, making a hexagon out of a lateral plate which is normally 

 pentagonal, seems to be an individual peculiarity of this specimen, for in 

 the case of other interambulacral areas in which the introduction of the 

 fourth column has been observed, the initial pentagonal plate abutted 

 against the initial plate of column 3, inducing an additional side on the 

 dorsal border of that plate, as in plate 2, figure 2. In one other case ob- 

 served (plate 3, figure 16), the initial plate of the fourth column did not 



* This is the specimen on which Mr Jagsjar based his observations of the development ^nd 

 arrangement of the interambulacrum. 



fits principal departure is in the slow rate of addition of columns 4 and 5 (compai'ed with plate 

 2, figure 2). Initial pentagon 4 is introduced considerably later than initial pentagon 3, instead of in 

 the next row, as in the figure cited. In area C of the same specimen, however, as shown in plate 

 4, figure 18, the fourth column begins in pentagon 4 at a very much earlier stage of growth than in 

 area yl. This different rate of development in two adjacent areas is uncommon and has never 

 been seen in any other specimen in the early added columns. 



