142 JACkSON AND JAGGAR — STUDIES OF MELONITES MULTIPORUS. 



difference of the number of columns is the basis of separation of the 

 genera. As Melonites ventrally has ambulacra like adult Oligoporus, and 

 as adult Melonites has much more complex ambulacra (plate 5, figure 

 20), it may be considered that Oligoporus is the more primitive genus 

 and Melonites the more specialized, being further advanced in the special 

 line of variation of the family. Melonites may be considered as highly 

 accelerated, for at an early period in development where Oligoporus (plate 

 6, figure 25), has but two plates in the ambulacrum, Melonites has four. 

 The four initial columns of Melonites, a a and b' b, are together equal to 

 the two columns of plates of less specialized genera, as the primitive Silu- 

 rian type Bothriocidaris, and also Archseocidaris, Cidaris, etcetera. This 

 relation is discussed in the succeeding paper under Oligoporus ("Arrange- 

 ment and Development of Plates in Oligoporus coreyi "), and is expressed 

 in a diagram, figure 1, inserted in the text at that place. 



The late honored Professor Sven Loven (27) has shown in modern echi- 

 noids that a considerable number of the ventral plates of the ambula- 

 crum are resorbed during the progressive enlargement of the actinostome. 

 More or less of this resorption has also probably taken place in the 

 growth of Melonites. These missing plates could only be obtained in 

 very young specimens. If present, they might show that the ambu- 

 lacrum in its inception had only two plates in a row, instead of four. 

 This would be in accordance with the earliest stages of Oligoporus, of 

 modern echinoids, and also with the condition of the adult in the ancient 

 primitive type Bothriocidaris and most members of the Pala^echinoidea. 



Structure and Development op the Interambulacrum. 



The interambulacra of the adult, when perfect, consist of two plates at 

 the ventral termination, as shown in three areas, A, Cand /, of plate 2, 

 figure 2. These plates, numbers 1 and 2, are pentagonal in form, having a 

 straight line on the ventral aspect. It is to be noted that the angles of 

 these plates do not correspond to the angles of terminal pentagons in later 

 added columns, as seen in this and other figures. These two first ventral 

 plates give rise, by the progressive addition of plates dorsally, to the two 

 outer columns of pentagonal adambulacral plates. In two areas, ^and 

 G of our specimen (plate 2, figure 2), there are three plates at the ter- 

 mination of the interambulacra. It will be observed that these plates 

 have angular faces ventrally, corresponding to the angles of the ventral 

 border of the SQCond row in an area terminating ventrally in two plates 

 (see also plate 2, figure 3). It is evident, therefore, that the lower row 

 of two plates is absent from mutilation in these cases, which at first sight 

 apparently terminated in three plates. This is an important point, for 



