ARACHNOIDES. 529 



actinal than on the abactinal side. In the buccal rosette the arrangement 

 is the same for both areas. Adjoining the actinostome the ambulacral fur- 

 rows are broad, lanceolate, but carry no pores in any part of the furrow. 

 The internal structure of the pillars is very different from that of Echina- 

 rachnius ; the walls are numerous, slender, frequently radiate or branch as in 

 Peronclla, though they retain a general parallelism to the outer edge ; they 

 extend nearly two thirds of the distance from the edge to the centre. We 

 have no solid interambulacral walls separating the interambulacral spaces, 

 so as to leave a broad bare triangular space, as in Echinarachnius. or as we 

 find it to a less decree in Mellita. The interambulacral walls are similar to 

 the ambulacral, arranged somewhat more fan-shaped ; the walls having, 

 besides the median free ambulacral spaces, three others (one median inter- 

 ambulacral and two lateral ambulacral), more or less distinct, separating the 

 interior into a number of triangular spaces ; the general arrangement of the 

 walls of each of which are repeated, or nearly so, in each ambulacral and 

 interambulacral space. 



The jaws are remarkably Hat, more like those of Echinodiscus than the 

 solid massive ones of Echinarachnius. The central part of the jaws is re- 

 markable for the high crest rising; on each side of the dentiferous furrow. 



The petals are diverging, sometimes even arched outwardly, so that, on 

 this account, and from its smaller width, the interambulacral space might 

 be mistaken for the ambulacral. The pores are conjugated to about 

 half-way between the apex and the edge, the petals extending no farther ; 

 the poriferous zone is continued in the same general direction by isolated 

 pairs reaching to the circumference. The ambulacral sutures running j)aral- 

 lel to the edge are riddled by pores, very much .as we find them in young 

 Clypeaster, and extend also to the ambulacral space, enclosed by the petal- 

 iferous part of the poriferous zone. The auricles are peculiar; they are two 

 independent, stout, short pillars, separated by a deep notch, instead of the 

 low. thin, indented auricles so common among the Scutellidae. 



The separation of Arachnoides from the Scutellidae seems to me unnatural ; 

 the whole of its internal structure (the teeth and partitions) points to a close 

 relationship with the Scutellidae, and not with the Laganidae, as suggested 

 by Midler, and accepted by Desor, with which it has only the straight 

 simple ambulacral furrows in common. 



