60 



longer than wide, and stand nearly upright, two pentagonal, two 

 hexagonal and two heptagonal. The plates in the third range are 

 about half the size of those in the second, and form an irregular 

 range. They are of unequal size and shape, and surround an elon- 

 gated, irregular, convex summit. 



The summit is covered by eighteen plates, fifteen of which are 

 quite small and some of them minute. There are six small open- 

 ings surrounding the summit beside the mouth. They are sup- 

 posed to represent the places for the attachment of the arms, and 

 adjoin the plates in the third range except one which seems to 

 be separated by a minute plate. The arm openings are in three 

 pairs. The mouth is surrounded by four plates, one of which be- 

 longs to the third rauge. It will, therefore, be observed that the 

 summit is covered with a number of small plates which are in no 

 manner connected with the orifices, and which preserve no definite 

 order of arrangement. The central one is surrounded with seven 

 plates, and the other ten plates have no order of arrangement, as 

 may be seen on figure 24. 



This species is so different from all others heretofore described 

 that no comparison is necessary with any of them. It furnishes 

 another illustration of the great diversity of forms belonging to a 

 single genus among the Cystideans. 



Fouud in the Niagara Group, at Louisville, Kentucky, and now 

 in the collection of Wm. F. E. Gurley. 



Family PLEUKOCYSTIDiE. 



PLEUROCYSTITES MERCERENSJS, n. sp. 



Plate V, Fig. 25, dorsal view slightly broken at the anterior end; 

 Fig. 26, basal view of same. 



Body irregularly subovate, and irregularly concavo-convex. 

 There are three plates on the dorsal side that do not extend to 

 the margin; each of these at the distal sides bears a pectinated 

 rhomb. The two anterior ones have the rhombs located near the 

 superior lateral margins, and the posterior one has the rhomb 

 near the column. The left anterior rhomb is the largest. They 

 are all somewhat elongate ovate, and have their longer diameters 

 directed toward a central point between them, and they are 

 striated longitudinally so that the strise are also directed in the 

 same way. The plates are radiately sculptured from the rhombs, 



