2 
CATALOGUE OF ECHIXIDA. 
a. Jaws distinct. 
Echinides paracentrosomes dentes, BlaAnv. D. S. N. 
Fam. 3. SCUTELLIDiE. 
Shell thick, pentagonal, elliptical, or circular, covered with 
small uniform bristles, supported bj very close, very small, 
very uniform tubercles on all parts of the shell ; mouth five 
sided; vent posterior, marginal, or infra-marginal ; jaws com- 
posed of five horizontal plates, pivoted on two pillars, which 
fit into two pits in their lower surface ; teeth simple, obliquely 
truncated, and inserted in a groove in the middle of the jaws ; 
internal cavity of the shell simple, or divided into compart- 
ments by perpendicular septa, or pillars ; ambulacra formed of 
single series of double pores, forming dorsal petals, and straight 
or anastomosary lines on the lower surface ; genital plates five, 
forming a circle round the madreporiform plate ; ocellar plates 
five, interposed between the genital plates and the apex of the 
ambulacra. 
Scutellidre, Gray , Ann. Phil. x. 1825, 5 ; Synop. Brit. Mus. 
1840; 1842, 115. 
Clypeastres, Agassiz , Prod. — Ann. Nat. Hist. i. 302 ; Deslong 
in Lamk. Hist. iii. 273. 
Les Echinides paracentrostomes dentes, Blainv. Diet. Be. Nat. 
lx. 195 ; Deslong in Lamk. Hist. ed. 2, iii. 271. 
Clypeasteroidea, y Scutellse, Agassiz; Bronn, Gesch. d. Nat. 196. 
Clypeasteroidea, Clypeastritse, Agassiz; Bronn , Gesch d. Nat. 
195. 
Mesostoma, part., Latr. Fam. N. R. A. 533, 1825. 
Clypeaster, Lamk. Byst. 349. 
Des Clypeasteroides, Agassiz & Desor , Ann. Sci. Nat. 1847, 129. 
The crustaceous covering of the body is usually thickened 
internally by an additional coat, and is strengthened by inter- 
nal columns, which enable them to resist the action of the sea 
for a length of time. 
The shells are formed of twenty bands of pieces, but the 
continuation of the ambulacral bands are often very much 
dilated and perforated. 
This family is allied to the Echinadce , by the nearly equal- 
sized spines, and having jaws, &c. ; to the annectant families 
of the order Btelleridce , by their jaws being only used for press- 
ing the food, and by the radiated lobed form of some of the 
species. 
