144 JACKSON AND JAGGAK — STUDIES OF MELONITES MULTIPORUS. 



Before describing further the plates of Melonltes it is desirable to con- 

 sider whether the lower row of two plates, numbers 1 and 2, plate 2, fig- 

 ures 2 and 3, really represent the first formed or initial interambulacral 

 plates. 



Professor Sven Loven (27) has shown that at the ventral border of 

 the interambulacra in the young of Goniocidaris^ and Strongylocentrotus 

 there is but a single plate which in the next row is succeeded by two plates 

 (plate 3, figure 8). At the same period of growth the ambulacra have 

 two columns of plates as in later stages. Of this, Professor Loven says, 

 " This is the normal constitution of the peristome in the whole class." 

 He shows that during later growth with the enlargement of the actino- 

 stome that area commonly encroaches upon the corona, and the initial 

 plate is gradually resorbed (plate 3, figure 9), and as the encroachment 

 of the actinostome continues, some succeeding binary plates are also re- 

 sorbed together with the corresponding i)ortions of the ambulacra. In 

 the Cl3^peastroids and Spatangoids the initial single plate is typically re- 

 tained throughout life, as shown in numerous figures published by Loven 

 (25), A. Agassiz (3), William B. Clark (6), and Duncan and Sladen (10). 



It is obvious that the enlargement of the actinostome may take place 

 by resorption of the plates of the base of the corona or by the progressive 

 growth of the same, both tending in the same direction, the enlargement 

 of the ventral circumference. Tiarechinus from the Trias is a genus wliich 

 retains the initial single interambulacral plate, and so does Lepldechimis 

 (plate 7, figure 42) and PholUlocidarh (plate 9, figure 54). 



From the above we gather the conclusion that resorption of ventral 

 plates may or may not take place, and that the interambulacrum prob- 

 ably always starts with a single plate. If Loven is correct in supposing 

 that the interambulacrum always terminates with a single plate, as every 

 evidence goes to prove, then Melonltes when young must have had a 

 single plate at the ventral border. 



In a specimen of Melonltes multlporas in Yale University Museum 

 (diamond number 157, specimen C) we find an important feature bear- 

 ing on the above consideration. In this specimen (plate 3, figure 10) f 

 the two ventral i)lates have each an angle toward the median line, and 

 these, together with the straight edge of the bottom, enclose a triangular 

 space which doubtless contained the single initial plate, as in a similar 

 stage of Strongylocentrotus plate 1' (plate 3, figure 9). This specimen of 

 Melonites does not actually show the initial single plate, and obviously 



* A veproduetion of Lov6n's figure of this genus is given as an insert, figure 3, in the chapter 

 entitled " Conclusions" of the succeeding paper; Studies of Pulteechinoidea. 



t In the specimen, plates 2 and 3 (see figure) have slipped down a little from their original posi- 

 tion, but they are restored to their places in the figure as they could be, all the angles being pre- 

 served, by simply moving them upward. 



