194 ECHINODERMATA—PELMATOZOA SUB-KINGDOM III 
the climax of Blastoid development takes place in the Sub-Carboniferous of 
North America. Some of the beds of the Kaskaskia Group are fairly charged 
with their remains, which as a rule are excellently preserved ; but above this 
horizon no traces of them have as yet been discovered. Nineteen genera, 
comprising upwards of 120 species, are recognised by Etheridge and Carpenter 
in their admirable monograph. 
Order1l. REGULARES. Etheridge and Carpenter. 
Pedunculate Blastoids with a symmetrical base, in which the radials and ambulacra 
are all equal and similar. 
Family 1. Pentremitidae. d’Orbigny. 
Base usually convex, and often much elongated.  Spiracles five, but sometimes more 
or less completely divided by a median septum, and bounded proximally by the wpper- 
most side-plates. Lancet-plate either entirely visible or partially covered by side-plates 
which extend to the margins of the ambulacra. Hydrospires concentrated at the lowest 
part of the radial sinus. Devonian and Sub-Carboniferous. 
Pentremites, Say. Calyx usually ovate or pyriform, with elongate, sub- 
truncate base. Ambulacra broad, sub-petaloid. Lancet-plate wholly exposed, 
and resting below on an under lancet-plate. 
Side-plates and outer side-plates numerous, 
the former abutting against the edges of the 
lancet-plates. Hydrospires three to nine ; 
spiracles single, or occasionally double; the 
two of the posterior side confluent with the 
anus, and forming with it a single large 
orifice. Summit covered by numerous spines, 
placed closely against one another so as to 
form a pyramid, which completely covers 
the summit and the greater portion of the 
spiracles. Excessively abundant in the Sub- 
Carboniferous of North America (St. Louis 
and Kaskaskia Groups), but not identified 
in Europe. P. Godoni, Defrance, and P. pyri- 
formis, Say, are the most familiar species. 
Pentremitidea, Orb. Calyx  clavate- 
ree antics ohare Lie pyramidal, with elongate, usually conical 
Sub-Carbon- Roem. Sub-Carbonifer. base, and truncate or convex upper face. 
(Nat size.) pect B Base mmit #8 Ambulacra narrow, short ; lancet-plate more 
or less completely concealed by side-plates. 
Deltoids very small, generally confined to the upper face of the calyx, and 
seldom visible in a side-view. Spiracles and hydrospires as in the preceding. 
Lower and Middle Devonian; Eifel, Ardennes, Spain, Great Britain, and 
North America. P. Pailletti, de Vern.; P. Eifelianus, Roemer; P. clavatus, 
Schultze. 
Mesoblastus, EK. and C. Calyx oval to globose, with concave to protuberant 
base. Radials long, deltoids small, short, unequally rhombic. Ambulacra 

Fic. 319. 
Fic. 320. 
