NATURAL SCIENCES OP PHILADELPHIA. 449 



ECHINODERMATA. 

 ASTERIDJE. 



Genus PALASTERINA, McCoy. 



Subgenus Schoenaster. 



The specimens of the beautiful star-fish, upon which we propose to found, 

 this subgenus, are unfortunately not in a condition to enable us to work out 

 fully the details of its structure. As far as can be determined, however, it 

 agrees in most of its characters with Palasterina, though it differs from the 

 typical species of that genus {Uraster primcevus, Forbes) in having the adam- 

 bulacral plates arranged with their longer diameter directed obliquely out- 

 ward, instead of at right angles to that of the rays. This oblique arrange- 

 ment, or lateral imbrication of these pieces, gives to each range, as seen from 

 below, a peculiar twisted or rope-like appearance, which suggested the name 

 Schoenaster (o-%oivis a rope, aVi-wg, a star.) 



Although we place it for the present as a subgenus under Palasterina, we 

 think it more than probable that when better specimens can be examined, it 

 will be found to present other differences of sufficient importance to entitle it 

 to rank as a distinct genus, in which case it can retain as a generic name that 

 by which we have designated it as a subgenus. 



Palasterina (Schoenaster) fimbriata. Body depressed, pentagonal, the 

 angles being extended into narrow, acutely pointed rays or arms, which are 

 convex above, and about equal in length to the diameter of the disk. Upper 

 side of the disk and arms composed of small, solid, convex, or somewhat 

 tumid plates. Ambulacral furrows deep, rather narrow, and bounded on each 

 side by the single row of oblique adambulacral pieces, which also form the 

 sides of the arms, beyond the disk, where some six or seven of these pieces 

 occupy a space of 0-35 inch. Disk apparently not provided with a regular 

 range of marginal pieces ; concave in outline between the rays, where it is, 

 like the sides of the arms, fringed by a single range of short, lanceolate spines. 



The plates forming the upper side of the rays near the disk, are hexagonal, 

 pentagonal, or irregular in form, and consist of about five or six ranges be- 

 tween the marginal rows, with a few much smaller intercalated pieces. 

 Farther out they gradually pass into two mesial ranges of oblong, alternating 

 pieces, arranged with their longer diameter parallel to that of the ray ; and 

 two series of much smaller, pentagonal or hexagonal alternating plates on 

 each side, between the middle ranges and the adambulacral rows. Towards 

 the extremities of the rays, these two ranges of small pieces on each side 

 diminish in size, and at last become obsolete, leaving only the adambulacral 

 and middle ranges. 



None of the specimens are in a condition to show the form and arrange- 

 ment of the plates forming the disk, nor the position of the vent and madre- 

 poriform plate. Near the extremities of the arms the dorsal pores, which are 

 comparatively small, pass chiefly between the ends of the oblong plates, form- 

 ing the two mesial ranges ; but farther in, towards the disk, they seem to be 

 somewhat irregularly distributed. There appear to be five bilobate, oral 

 pieces, but we suspect each of these is divided by a close fitting suture, so as 

 to make ten in the entire series. 



Greater diameter, 2-37 inches ; lesser do., 0-89 inch ; breadth of ambulacra] 

 furrows, about 0-10 inch ; length of marginal spines, 0-07 inch. 



Locality and position. St. Clair county, Illinois. St. Louis Limestone of 

 Lower Carboniferous series. 



I860.] 



