124 BRITISH PALASOZOIC FOSSILS. [ EcutnopERMata. 
Position and Locality—Rare in the carboniferous limestone of Derbyshire. 
Explanation of Figure.—Plate 3. D. fig. 9. View of specimen from Derbyshire, magnified four diameters ; 
the line shews the natural size. 
PeENTREMITES DERBIENSIS (Sov.). 
Ref—Sow. Zool. J. Vol. Il. t. 11, f. 3. Phil. Geol. York. t. 3. f. 10. 
Sp. Ch.—Spheroidal or slightly pyriform, greatest width above the middle, upper part obtusely rounded, 
lower portion slightly tapering to the concave, pentagonal base, the angles of which are prominent; section 
obscurely pentagonal, each of the sides slightly flattened in the middle, and slightly convex at the margins; 
re-entering angles of transverse suture slightly less than one-third of the length from the base ; surface sha- 
greened with obscure rows of close minute obtuse granules; average length six lines, width the same (or 
a little more in many specimens). 
This species varies a little in shape, the lower half of some specimens being more tapering, and the upper 
part more inflated than in others; in some the base is unusually narrow, and the width varies considerably. 
The transverse suture being considerably below the middle separates it from the P. orbicularis, although certain 
lines occasionally visible in old inflated specimens, on the second series of supra-basal plates, parallel with the 
transverse sutures, might easily be mistaken for them if care be not taken to observe that the vertical suture is 
not continued above the basal third of the length. 
PENTREMITES ELLIPTICUS (So7.). 
Ref.—Sow. Zool. J. Vol. I. t. 11, f. 4. Phil. Geol. York. t. 3, f. 6, 7, 8. 
Sp. Ch.—Elliptical, greatest width about the middle, gradually narrowing to the obtusely-rounded apex, 
and narrow, pentagonal, concave base ; section obscurely pentagonal, the sides slightly convex in the middle, and 
very slightly concave towards each end; re-entering angle of transverse suture midway between the apex and 
base; surface closely covered with obscure lines of minute granules: average length six lines, in proportion 
thereto the greatest diameter is =, width of base ;. 
In size and shape this resembles the P. ob/ongus (Gilb.), from which, however, the mesial transverse suture 
easily distinguishes it. 
Position and Locality—Not uncommon in the carboniferous limestone of Derbyshire. 
5th Ord. PrrRISCHOECHINID®. 
Body encased in a spheroidal crust, of more than twenty rows of strong calcareous plates ; five ambulacra, 
composed of two rows of pentagonal plates each, extend from the mouth in the middle of the ventral aspect 
to the anus on the middle of the superior or dorsal aspect, pierced by two double rows of ambulacral pores, and 
terminated above by a pentagonal ocular plate; interambulacra terminated dorsally by five large pentagonal 
ovarian plates, the posterior one of which is larger than the rest and porous; rows of interambulacral plates 
three, five, or more, the outer or lateral rows pentagonal, the inner ones hexagonal. 
I have established this Order (Ann. Nat. Hist. 2nd Series, Vol. III. p. 251) for the reception of those 
Echinodermata of the Palzeozoic Rocks hitherto confounded with the Hchinide ; all of the latter Order how- 
ever agree in having the case made up of twenty vertical rows of plates, five pair of ambulacral, and five pair 
of interambulacral ones; this is not only the most persistent character of the entire group, but the number 
becomes of great interest when, with Agassiz, we view the globose test of the sea-urchin as a mere modification 
of the parts of a five-rayed star-fish,—an ideal division of the medial suture connecting the two rows of plates 
in each interambulacrum of the former, giving at once the ambulacra, lateral ossicles, and other characters of 
the latter. The Kchinites of the Paleozoic rocks however are constructed on an entirely different plan, having 
three or some greater odd number of rows of interambulacral plates, instead of ¢zo as in those of the newer 
