390 Dr. H. A. Nicholson and Mr. R. Etheridge on 



In order to examine the corallites in transverse section it is 

 necessary to cut a slice tangential to the upper surface 

 (slices cut tangential to the lower surface, owing to the deflec- 

 tion and approximate horizontality of the marginal corallites, 

 exhibit oblique longitudinal sections of the tubes). In a pro- 

 perly prepared transverse section (fig. C), the corallites are 



A, under surface of a small specimen of Prasopora GraycB, natural size, 

 showing the epitheca. B, side view of another, larger specimen, of 

 the natural size. C, transverse section of the same, enlarged about 

 twenty diameters. D, longitudinal section, similarly enlarged : a, 

 one of the corallites ; b, one of the ccenenehymal tubes. The dark 

 shading in the microscopic sections indicates where the matrix is 

 present. 



seen to be of a rounded or more generally hexagonal figure, 

 with thick and well-defined walls. In the centre of each is a 

 rounded or oval opening, representing the section of the cen- 

 tral tube of the corallite, filled either with calc spar or with the 

 dark matrix of the enclosing rock. This opening is sur- 

 rounded by one or more curved lines, which are often tangents 

 to the margin of the central aperture, or are concentric to it, 

 or intersect one another. These lines are the cut edges of the 

 tabulae which form the exterior zone of each corallite ; and 

 they forcibly remind one of the somewhat similar appearance 

 presented by transverse sections of the corallites of Syringo- 

 pora^ with their infundibuliform tabulae. The corallites, in 

 fact, in transverse section present a general resemblance to the 

 bulb of an onion when cut across ; and our generic name (Gr. 

 prason, a leek) is in allusion to this. The central tube of the 

 corallites is about one third or often more of the total diameter ; 

 and tlie walls of the corallites are destitute of mural pores, and 

 are not fused with one another or with the tubes of the coenen- 

 chyma. 



