DESCRIPTION OF MELONITES SEPTENARIVS. 183 



plates. It is a small species, being one of the smallest known. It is not 

 perfect ventrally ; but the height of the test as far as shown is 4 centi- 

 meters. 



The ambulacral plates are not well preserved, but can be made out in 

 places. In the ambulacrum on the right of the figure there are 4 col- 

 umns of plates at the ambitus in a half area. This shows that the species 

 is characterized by 8 columns of ambulacral plates — an unusual number 

 in the genus. Two pores exist in each plate. The width of the ambu- 

 lacrum at the ambitus is 1.3 centimeters, narrowing toward the dorsal 

 area. 



The interambulacrum ventrall}^ has, as far down as preserved, 6 col- 

 umns of plates. A seventh column is introduced b}^ the pentagonal plate 

 7, with a heptagonal plate, H^ on its left. This seventh column at its point 

 of origin is remarkable in that there are 5 columns on the left of it and 

 only 1 on the right. Odd-numbered columns commonly originate in a 

 median position, with an equal number of columns on either side. Rarely 

 exceptions are found, as shown in the tables of Melonites multiporus (pages 

 165-170), in which, in several cases, an odd-numbered column origi- 

 nates to the right of the center, with one more column on the left than 

 on the right ; but no case has been observed in any Palseechinoid other 

 than the present one in which Siny greater degree of irregularity existed. 

 This interambulacrum is also peculiar in that a plate, P, normally hex- 

 agonal, is pentagonal in outline, and an adjacent plate. A, is heptagonal. 

 No other case of irregularity quite like this has been seen in any of the 

 Melonitidse. Toward the dorsal termination of the area the interambu- 

 lacral plates make an approach to the rhombic form seen in other species 

 (plate 3, figure 13 ; plate 5, figure 21 ). The width of the interambulacrum 

 at the ambitus is 2.3 centimeters. The interambulacrum is quite elevated 

 in sectional outline, but presents a continuous curve rather than almost 

 an angle on its lateral borders, as in Melonites giganteus (plate 4, figure 19). 



The nearest ally of Melonites septenarius is Melonites indianensis, Miller 

 and Gurley^(34). It differs from that species in the proportionately 

 much narrower ambulacra, in having 7 instead of 6 columns of interam- 

 bulacral plates, and in the gently curving, rather than strongly melon- 

 like form of the corona. 



The number of columns of ambulacral plates is an important feature in 

 classification, and the question comes up whether species having eight 

 columns, as Melonites indianensis and septenarins, should be separated ge- 

 nerically. We think not, because in the genus Melonites the number of 

 columns of ambulacral plates is quite a variable feature, as shown in the 



♦The type of this species is in the private collection of Mr Wm. F. E, Gurley, of Springfield, 

 Illinois. 



