FROM THE CARBONIFEROUS LIMESTONE OF IOWA. 593 



It presents somewhat the appearance of a street 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. 



Perisomic 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 super ior 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 pcrisomic 

 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. 10, a, b.) 



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 subcorneal, 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 pcrisomic plates. 



The pcrisomic 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 striae, running- 

 parallel to the lateral and inferior borders. These striae 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. Each 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 TJNICORNUS. (N. S.) 

 (Tab. V. A, fig. 12, b.) 



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 arc hexagonal, and one is irregular and heptagonal. The second scries 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 scries ; 

 four are nine-angled, one being situated between each pair of quadrangular plates, and their inferior salient 



