104 



parts. It may, for the sake of convenience in noticing these 

 variations, be said to be composed of three parts, a central 

 or depressed portion, crowded with cells, a raised or pro- 

 minent circle round the centre, formed of erect or semi-erect 

 aggregated tubes, and a thin, plain, translucent margin des- 

 titute of cells. 



In form it very much resembles a miniature soup-plate. 

 It is calcareous, white, sessile, rarely exceeding half-an-inch 

 in diameter, and is attached by the base of the central de- 

 pression. The depression varies in size from a mere point 

 to about three-eighths of an inch in diameter, and is either 

 round, or very irregularly oval. The surface is occupied 

 by small, semi-horizontal, closely aggregated tubes, with 

 oblique even apertures. The raised circle is also liable 

 to great variations in form. In most cases the tubes rise 

 abruptly from the circumference of the depression, form- 

 ing well defined fluted sides to the cup ; sometimes they rise 

 in gradual succession one above another, forming a gently 

 sloping surface from the margiu to the centre; sometimes 

 the surface is arched concavely, at others convexly, and 

 between these extremes there is every possible variation. 

 The sides of the cup are generally uniformly fluted, a furrow 

 being formed by each tube ; but sometimes the tubes are 

 arranged in companies, and then, the sides appear formed of 

 fluted columns. The tubes are of equal diameter and lean 

 in a greater or less degree, obliquely outwards. The aper- 

 tures are generally on one plane, but occasionally rise in 

 radiating ridges; but some will rise singly above the others. 

 The apertures are even and oblique ; but sometimes, if the 

 specimen be examined as soon as it is taken, the apertures 

 appear armed with one or two spines, but this appears to me 

 the result of the greater growth of one side of the lip than of 

 the other, for their situation is by no means constant, they 

 are often absent, and always form a continuation of the sides 

 of the tubes. The third or external part is frequently 

 wanting. In its greatest developeinent it is about as wide as 

 the middle or raised circle of tubes; it is calcareous, trans- 

 lucent, very thin and radiatingly striated with the rudiments 

 o* tubes, which are thickly arranged, though never in contact. 

 They however sometimes branch and anastomose with each 

 other. In a very recently formed portion of this margin, 

 these tubes do not extend quite to the circumference, but as 

 age advances they get more and more elongated till the whole 

 surface is permeated by them ; so that at first they appear 

 pointed but afterwards of equal diameter throughout. That 

 portion of the surface which is next the circle of tubes is 

 always occupied by partially formed apertures of new tubes, 

 and from examining large numbers of specimens from all 



