OWEN AND SHUMARD'S FOSSIL CRINOIDEA. 67 



Pentremites stelliformis. (New sp.) 

 Fig. 16, a, b. 



The body of this pentremite is much less elongated than the preceding species, 

 and its diameter is proportionally greater. It is moderately convex at the summit 

 and depressed conical below. 



Basal plate pentagonal, depressed sub-conical, slightly constricted near the middle 

 by a circular depression, 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 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 stelliform 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 

 sub-carboniferous limestone at Burlington, Iowa. It is rare, the specimen figured 

 being the only one that we have discovered. 



ACTINOCRINUS UNICORNUS. (NeW Sp.) 



Fig. 12, a. b. 



General figure depressed, sub-conical below the insertion of the arms, convex at 

 the summit. 



Basal plate tripartite, somewhat massive, sub-octagonal, bordered by a rim more or 

 less elevated, its continuity interrupted by three triangular notches. Six somewhat 



