SUB GENUS PRASOPORA. 207 



conical, with a flat base, the amount to which the margins 

 are extended varying in different examples. The base may 

 be flat or concave, and is covered with a concentrically striated 

 epithecal plate, while the upper surface is covered with the 

 calices. 



As regards internal structure, the corallum is made up of 

 two kinds of corallites, larQ^e and small. The larcre tubes are 

 provided with perfectly distinct walls, and are oval or irregu- 

 larly circular in shape, and from i-50th to i-6oth inch in 

 diameter. From their shape, they are only in contact at 

 limited points, and the interspaces between them are filled 

 with the small corallites, the amount and number of these 

 beino- somewhat variable. The small tubes are anovular or 

 sub-angular in shape, with a diameter of i-i50th inch to i-200th 

 inch, and at intervals are collected into stellate groups or 

 " maculae." The tabulae of the large tubes are of two kinds, 

 being mostly convex, and so arranged as to form a series of 

 convex vesicles on one side of the visceral chamber, whilst 

 others run straight from the preceding to the opposite wall, 

 or cross the tube directly from side to side. The tabulae of 

 the small corallites are complete and horizontal, and are very 

 numerous and close-set. 



Obs.- — This species was formerly (Pal. Tab. Cor., p. 316) 

 confounded by me with M. Whiteavesii, Nich., as I have 

 explained in speaking of the latter form— an error which will 

 appear not unnatural when it is remembered that the tvvo 

 species occur in the same formation and at the same locality, 

 and have a general similarity to one another in their shape and 

 mode of growth. M. Selwynii, however, so far as I can judge 

 from the specimens which have been actually determined by 

 microscopic examination, is, in its type-form, of much larger 

 size than M. Whiteavesii, while the internal structure of the 

 two is extremely different. 



Tangential sections of M. Selwynii (fig. 44, b and c) show 

 that the corallum is composed of two quite different sets of 

 corallites, which are not only different in size, but also in 



