CAMBRO SILURIAN MICR0-rALvT30NT0L0GY. 1% 



PRASOrORA OCULATA. (N. Sp.) 



riato in., figs. Uff, 



Zoarium froo, or attached, discoid, concavo-convex, sharp-edged in the 

 immature state, more or less obtuse in the adult ; varying, in the four 

 specimens examined, from 15 to 30 mm. in diameter, and from 2*5 to 3*5 

 mm. in thickness in the centre. Under surface covered with a thin 

 smooth epitheca, which exhibits in a young example two or three con^ 

 centric wrinkles. Upper surface conspicuously marked with irregularly 

 rounded, or sub-polygonal depressions, generally a little less than their 

 own diameter apart, In the centre of each of those depressions there is a 

 small somewhat compressed elevation slightly raised (as seen in profile) 

 above the surface of the zoarium. The latter is made up of two kinds of 

 tubes, the larger of which can bo seen with a hand-lens on a well-preserved 

 specimen. The smaller series of tubes which fill up the spaces between 

 the larger ones can only be discerned in microscopic sections. Of the 

 larger tubes some exceed the average in size, and appear to occupy tho 

 depressed areas of the surface of the zoarium ; from three to four of thom 

 are contained in the space of 1 mm., while from four to five of the smaller 

 or average sized cell-apertures fill a like space. 



Tangential sections shew the larger tubes to be irregularly circular, or 

 polygonal in outline, and usually completely isolated from each other by 

 the smaller tubes. These latter are angular or sub-angular in form, and 

 variable in size ; here and there clusters of them form stellate groups or 

 macula) surrounded by a set of tubes larger than the average ; among tho 

 latter but few of the smaller sized angular tubes penetrate, so that in 

 many places their walls are completely in contact, leaving only small 

 interspaces at their angles occupied by the smaller angular tubes. Tho 

 little elevations situated in the centre of the depressed areas appear to bo 

 made up of the last named tubes. 



In longitudinal sections this species exhibits remarkable characters. 

 Tho larger tubes are furnished with cystoid diaphragms which are alter- 

 nate in their arrangement on either side of the visceral chambers ; these 

 diaphragms are usually of a conical form, and sometimes narrow and 

 pointed, they vary much in their distance from each other ; in some 

 places they are separated by a space equal to a tube-diameter, in others 

 they are closely approximated; occasionally a straight diaphragm unites 

 them with the opposite wall of the tube. There arc a few straight hori^ 



