FROM THE CARBONIFEROUS LIMESTONE OF IOWA. 593 
Tt presents somewhat the appearance of a strect lamp, its lower portion having the form of a truncated 
pentagonal pyramid inverted, while its upper part is dome-shaped. 
The basal plate is composed of three pieces, two are widened pentagons, and one is a rhomb; united 
together, they form an irregular octagon with three obtuse re-entering angles, to the points of which the 
basal sutures radiate. 
Perisomice plates five, elongated ; their length being to their breadth about as one to two; three of 
them terminate inferiorly in an obtuse salient angle, each angle corresponding to one of the retreating 
angles formed by the union of the basal pieces, two terminate below in a straight edge, and rest on the 
superior margins of the pentagonal basals. A triangular fold projects exteriorly from the superior por- 
tion of each plate. The apex of this triangle points downwards and extends nearly to the middle of the 
plate. These folds are notched superiorly for the reception of the five radiating ambulacra, and a portion 
of the superior plates, and on either side is a short, straight edge. 
The superior plates are dart-shaped, the barbs being inserted into the triangular notch on either side 
of the ambulacra, while the intervening edges rest on the corresponding straight margins of the perisomic 
plates included between the folds. The point of the dart terminates at the apex of the fossil. The 
ambulacra widen slightly from below upwards. 
This Pentremite was obtained from the carboniferous strata on Mill Creek, Randolph County, Illinois, 
about seven miles from Chester. 
PENTREMITES STELLIFORMIS. (N. S.) 
(Tab. V. a, fig. 16, a, 2.) 
The body of this Pentremite is much less elongated than the preceding species, and its diameter is pro- 
portionally greater. It is moderately convex at the summit and depressed conical below. 
Basal plate pentagonal, depressed subconical, slightly constricted near the middle by a circular depres- 
sion, and composed of three pieces, two pentagonal and one quadrilateral: the upper edges are nearly 
straight, and support the five perisomic plates. 
The perisomic plates are short and much elevated in the centre, each plate being in fact a four-sided 
pyramid rounded at the angles, and having its apex projecting exteriorly. These plates increase slightly 
in width from below upwards, and their superior margins are obliquely truncated on both sides of the 
ambulacra for the reception of the superior pieces. The surface is covered with fine strie, running 
parallel to the lateral and inferior borders. These strie are so fine that they are scarcely perceptible to 
the unassisted eye. The union of these pieces forms at the summit a decagon, very regular, in which five 
angles are salient and five retreating. Five radiating petalloid ambulacra commence at a central stelli- 
form space, and terminate near the tips of the salient angles of the decagon. Lach of the ambulacra are 
divided into two parts by a median furrow, and each part is made up of a great many small articulations, 
the form and arrangement of which cannot be determined in our specimen. : 
The superior plates are small and lie nearly horizontal: four of them are elongated, eight-sided and 
irregular ; the fifth is larger than the rest, and composed of two pieces of an irregular form united together 
transversely, and perforated by an oval opening at the suture. 
This neat and elegant Pentremite occurs with various other crinoidal forms in the subcarboniferous 
limestone at Burlington, Iowa. It is rare, the specimen figured being the only one that we have dis- 
covered. 
ACTINOCRINUS UNICORNUS. (N. S.) 
(Tab. V. a, fig. 12, a, 5.) 
General figure depressed, subconical below the insertion of the arms, convex at the summit. 
Basal plate tripartite, somewhat massive, suboctagonal, bordered by a rim more or less elevated, its 
continuity interrupted by three triangular notches. Six somewhat massive plates, tumid in the centre, 
surround the basal plate, five are hexagonal, and one is irregular and heptagonal. The second series con- 
sists of twelve pieces, five are quadrangular, short (their height being to their breadth as one to two), 
slightly arcuated on their edges, and rest on the upper edges of the hexagonal plates of the first series 5 
four are nine-angled, one being situated between each pair of quadrangular plates, and their inferior salient 
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