CATALOGUE OP ECHINIDA. 
39 
thin, and formed of twenty bands of pieces, like all the other 
Eckinida , but the interambulacral arese are unequal: the 
posterior hinder ones are usually very broad, the widening of 
which is formed more especially by the extension of the pieces 
of the posterior bands of this area ; the hinder middle area is 
rather irregular, the posterior series of each of the two hinder 
ambulacra being extended into it just below the anus, so as to 
form an isolated subpentagonal piece (the subanal plate), ex- 
ternally marked by a smooth groove ; the ambulacra are thin, 
being extended beyond it, leaving a central ovate or lanceolate 
medial inferior area (called the plastron) ; round the mouth 
there are five grooves, the continuation of the ambulacra, 
which are more or less perforated with holes, through which 
pass out branched tentacula, somewhat like those of Holothuria. 
See Leske, t. 43, f. 5; Gray, Ann. Phil. 1825. 
Van Phelsum, in 1774, used the fasciole (under the name of 
, “ Margines multangula ”) to characterize this genus Nuces (the 
Brissus of Klein). 
Dr. Fleming used the band of minute spines on the surface 
as a character to separate the species into sections ; thus : — A 
I svJbquadrrangular space on the vertex, containing the orifices 
of the oviduct inclosed by a narrow band ; the pairs of pores 
in the avenue not connected by lines ; compressions at the 
vent vertical. B. cordatus and S. ovatus. B. Destitute of a sub- 
quadrangular space on the vertex ; the pairs of pores in the 
avenues connected by transverse lines ; compression at the vent 
oblique. S. purpureus. British Animals, 1828, p. 480. 
Desmoulin ( Etudes sur les Echinides, 1835, p. 55) refers to the 
! fascioles under the name of a linear impression, which he com- 
pared to the pallial impression in the shells of bivalves. He 
proposed to divide the Syatangi into three sections by them, 
I thus : — a. Dorsal impression placed on the crown between the 
apex of the ambulacra, B. arcuarius. b. Dorsal impression sur- 
, rounding the excavated part, or petaliform ambulacra, B. 
ovatus. c. Dorsal impression none, B. purpureus. 
MM. Agassiz and Desor {Ann. Bci. Nat. 1847) have more 
extensively applied this character, and used it to define the 
generic characters. 
The position of the fascioles or bandelets varies in the different 
genera ; those which surround the ambulacral petals are called 
peripetalous, as in Hemiaster and Schizaster, &c. ; when they sur- 
round the odd ambulacra, internal, as in Amphidetus; when 
they extend in front and behind on the sides, as in Bchigaster, 
lateral; when limited to the base of the vent, subanal. The 
