80 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
Compsocrinus, — both from the Lower Silurian, — have uniserial arms; Peri- 
glyptocrinus, and probably all later Melocrinide, biserial. Among dicyclic 
Camerata, we find well defined biserial arms already in the Trenton and 
Hudson River groups, along with uniserial, the former being perhaps in the 
majority ; but the latter are continued to the lower part of the Devonian by 
two species,* and these, together with the species of Dichocrinus, are, so far 
as we know, the only representatives of the Camerata with a single row of 
arm plates that survived the close of the Silurian. 
In the Fistulata, the biserial arm structure was introduced just before the 
close of the Carboniferous, but only in a limited way. ‘True biserial arms 
only occur in Graphiocrinus,, Hupachycrinus, Cromyocrinus, Hydreionocrinus, 
Evisocrinus, Stemmatocrinus, and Encrinus, but the majority of species have 
either quadrangular or cuneate arm plates, and in some of them only the 
tips of the arms begin to interlock. In this group the biserial stage at no 
time became a constant character, not even in the Triassic. Enerinus hihi- 
formis has perfectly biserial arms, while the arms of Huerinus gracilis are 
uniserial, and composed of quadrangular plates. 
The arms of the Articulata, not only in Paleozoic pinnuleless forms, 
but also in the Neozoic pinnule-bearing ones, are uniserial without exception. 
The pinnules, as happily expressed by Carpenter, are repetitions of the 
arms on a small seale; and in their organization morphologically, and to 
a large extent physiologically, closely resemble ordinary arm_ branches. 
They are short branchlets given off along the sides of the arms, but rarely 
reaching their tips, and are usually more slender, and composed of longer 
joints. The pinnules differ from arms in containing the fertile portions of 
the genital glands, while the arms lodge the genital cords. Like the arms, 
they have ambulacral furrows fringed with cilia, by means of which particles 
of food coming in contact with them are carried along the grooves to the 
* Oehlert’s new genus Diamenocrinus, and “ Rhodocrinus” gonatodes Miller (both from the lower 
Devonian), which perhaps belong to one genus, have dichotomizing arms, composed of short, quadrangular 
plates. 
+ It is doubtful if the name Graphiocrinus ean be retained, according to the general rules adopted by 
naturalists, as it was incorrectly defined by de Koninck and Le Hon. The type has small infrabasals hidden 
by the column, and the position of the anal plate is materially different from that given by the Belgian 
writers, which probably was not known to Trautschold when he proposed the genus Pdialocrinus. The anal 
plate of Graphiocrinus encrinoides, de Koninck’s type, rests directly upon the truncated posterior basal, as 
we observed in a fine specimen in our collection, exactly as in Phialocrinus. There is, however, a slight ob- 
jection to Trautschold’s name, which was preoceupied by Hichwald (Lethea Rossiea I, p. 578), but the 
genus was founded merely upon fragments of column. That Phialocrinus patens has two costals, and Graphio- 
crinus encrinoides but one, may not be of generic importance, as the two plates of the former are equal to the 
one of the latter, which evidently form a syzygy. 
