34 SVEN LOVEN ECHINOLOGICA 



oeps. There seems to exist some linear arraiigemeiit leading 

 towards a group of larger verruculated lamels near tlie pri- 

 niary pair. — The Saleiiia Pattersoni Al. Ag., ftg. 160, pre- 

 .sents a peculiar disposition.^ The primary ambnlacrals are very 

 large and have their j)ores near the aboral margin. Between 

 them and the peristome there is a succession of large imbricated 

 scales arranged in three series, one mesial and two lateral, 

 while the interradial areas are covered by a series of their 

 own, of smaller scales. 



It is not here the place to enter into further details re- 

 garding the form and distribution of these minute particles, 

 derived, for a great part at least, from the disintegration of 

 the first peristome. 



In the whole dass of the Echinoidea it is the rule tliat 

 the ambulacra constitute a double column of paired piates con- 

 tinuous from the lips to the radials of the calyx. So it is in 

 the SpatangidiP,- so in the Echinoneus.^ In the Cassidulidai* 

 the priniordial ambnlacrals are very large. In the Clypea- 

 stridcE-^ they were recognized already by Johannes Muller,^ 

 conspicuous as they are from the size of their pores and pe- 

 dicels. 



Likewise it is a rule that the columns of the interradia 

 begin ventrally each with a solitary plate. This is univer- 

 sally so in the Spatangidfe, the Cassidulidffi and the Clypeas- 

 tridee; the Echinoneus, notwithstanding the distortion of its 

 peristome, preserves the solitary plate entire, or nearly so, in 

 the interradia 2, 3, 5. In all these groups it forras part, in 

 the adult, of the first circle. 



The Ectobranchiates, inchiding the Echinoconidee, and the 

 Cidaridse, do not fail strictly to obey both these rules. At an 

 early period of their nymphal life each priniordial pair of the 

 ambulacra is closely followed by its two columns, and the in- 

 terradia begin each with a perfect solitary plate, Pl. II, fig. 

 .9, Pl. IV, fig. 25. Bnt at the same time as the skeleton is 

 being built up thus in accordance with the general rule, an- 



' Al. Agassiz, Blake Ecliinoidfa, p. 14, Pl. IV, fig. 4. 17. 19. 

 2 Etudes, Pl. XXIII— XLII. 

 ' Ib., Pl. IX. 



* Ib., Pl. VII, XXII. 

 5 Ib., Pl. XLIV— LII. 



* Ueber den Bau der Echinodermeii, p. 39. 



