514 A POPULAR EXPOSITION OF THE 
Pentremites (fig. 84) is the principal. It is chiefly 
characteristic of the Devonian and Carboniferous for- 
mations. A closely related form—separated generically 
under the name of Blastoidocrinus*—has been des- 
cribed by Mr. Billings from the Chazy limestone of the 
Trenton Group, a member of the Lower Silurian series, 
(Canadian Organic Remains: Decade IV.) Fig. 84. 
3. Cystidea—The representatives of this Order are more or less 
closely allied to the crmoids. The cystideans possessed a globular or 
oval body attached to the sea-bottom by a short stem. The body 
was covered by polygonal plates, which in some genera were arranged 
in definite order, and in others, irregularly. 
Arms were either rudimentary or altogether 
wanting. The body openings were three in num- 
ber, comprising (according to the more general 
view) an oral, anal, and ovarian aperture. The 
latter (or according to some paleontologists, the 
oral orifice) was surrounded by five or more 
triangular plates, forming a kind of pyramid. 
In addition to these openings, most genera 
exhibit a series of pores, either distributed ir- 
regularly over the body-plates or collected into 
lozenge-shaped areas termed “‘ pectinated rhombs, ” 
see ane 85 (= Glyptocystites Logani, Billings). 
Fig. 85. 
The cystideans were limited entirely to the Silurian period. Not a 
trace of this Order is found in the rocks of any succeeding epoch. 
Various species, but mostly in a very fragmentary state, occur in our 
Canadian strata. These are illustrated and described by Mr. Billings 
in Decade III. of Canadian Organic Remains. The following is an 
analysis of the leading forms, extracted from a review, by the writer of 
this Essay, in the Fourth Volume of the Canadian Journal (New 
Ser 
“With regard to the Lower Silurian species of Canadian cystidex, Mr. Billings 
describes nineteen new forms, belonging to his genera, Pleurocystites, Glypto- 
* Pentremites exhibits three series of plates (exclusive of the Ambulacroid series): Basals 
Radials, and Inter-radials, the latter resting upon the radials in alternate position. The 
radials are comparatively large, the inter-radials small, so that the ambulacroids extend 
nto the former, In Blastoidocrinus the reverse of this takes place. The inter-radials are 
large, and the ambulacroids do not extend below them. 
