NATURAL SCIENCES OF PHILADELPHIA, 3S3 



The first arm pieces are much larger than the others, and consist of a single 

 series ; but above these the arms soon pass into a double series of small 

 alternating pieces, which are considerably wider than long, and support 

 on the inner side a double series of closely arranged tentacles. The 

 arms, of which there are four to each ray, or twenty in the entire series, 

 are rounded on the outside, and deeply grooved within, slightly taper- 

 ing, apparently rather long, and entirely simple. The interbrachial appendage, 

 rising from the middle of each group of arms, is thin or knife-like, and 

 seems to have been nearly as long as the arms. 



Locality and position. Hardin County, Illinois. Chester Limestone, of sub- 

 carboniferous series. 



Dichocrinus (Pterotocri.vos) Chesterensis. Body below the arms rather 

 small, basin-shaped, or more than twice as wide as high, widening rapidly up- 

 wards from the base, the sides being slightly concave in outline ; composed of 

 moderately thick, apparently smooth plates, which are connected by linear 

 sutures. Base more than one-third as broad as the body, about three times as 

 wide as high, truncate and concave below, the concavity being margined by an 

 angular rim; columnar facet small, or less than one-third as broad as the trun- 

 cated under side of the base. First radial plates about the size of the basal 

 pieces, though proportionally higher, twice as wide as long, and widening 

 rather rapidly from below ; four of them quadrangular, and one on the anterior 

 side, pentagonal; all distinctly concave on the upper side, (which is longer 

 than either of the others) and having their salient lateral angles above slightly 

 truncated, apparently for the reception of small interradial pieces. Second 

 radial pieces minute, or merely rudimentary, triangular, and each partly 

 supporting on its sloping upper edges two larger secondary radial pieces, 

 which also rest with one side on the primary radials, and bear upon their 

 superior sloping sides the first brachial pieces, the outer of which, like- 

 wise have one edge reposing upon the lateral superior edge of the first radial 

 plates. Anal piece about one-third as large as the primary radials, ovate in 

 form, the upper extremity being very narrow, and the long lateral margin 

 convex, while its base has an obtuse angle on each side, and one in the middle. 



The arms, of which there are four to each ray, or twenty in the entire range, 

 are simple from their origin, rounded on the outside, deeply grooved within, 

 and commence as a single series of larger pieces, but soon pass into a double 

 alternating series of small pieces, which support on their inner sides two ranges 

 of tentacles. The interbrachial appendages, if there are any in this species, are 

 unknown. 



Height from base to summit of first radial pieces, 0-16 inch ; breadth at the 

 top of first radials, 0-40 inch ; breadth of base, 0-24 inch ; height of do., 0-16 

 inch; length of anal piece, 0-15 inch ; breadth of do., 0-09 inch. 



Locality and position. Chester, Illinois. Chester Limestone of the subcar- 

 boniferous series. 



Genus TREMATOCRINUS, Hall, I860. 



Trematocrinus fiscellus.* Body short, subcylindrical, slightly longer than 

 wide, truncated and concave at the base, and nearly flat above ; sides rising 



* In first sending on the description of this species to the Academy, we had proposed 

 to found upon it a subgenus, which we placed provisionally under Aoanthocj inus, stating, 

 at the same time, that it presented differences from the type of that genus, that might 

 be of full generic value. As this paper is going through the press, we have received a 

 copy of Prof. Hall's Supplement to the Iowa Report (just published), in which we see he 

 has described four species of this type under the name of Trematocrinus. We there- 

 fore cancel, in lhe proof of our paper, the description of the group, and refer our 

 species, which is clearly distinct, specifically, from the forms described by Prof, Hall, 

 to his genus Trematocrinus. 



In our remarks (now cancelled) on this group, we had pointed out its relations not only 

 to Acanthocrinus, but also to Rhodocrinus, Miller, and to Goniasteroidocrinus of Lyon 

 and Casseday. Its analogy to the latter is so striking, that we strongly suspect the same 



1880.] 



