a new Genus and Species of Silurian Corals, 389 



corallites, prismatic, usually arranged in a single zone round 

 each coraliite, and crossed bj numerous complete tabula. 

 Corallites and ccenencliymal tubes in contact with one another, 

 but not soldered together by their walls. 



Obs. Tlie corallum in this genus is composite, and has the 

 general form and aspect of the massive species of ChcBtetes 

 {Monticulipora) . Its general shape (fig. B) is concavo- 

 convex or hemispherical, the concave or flat base being 

 covered by a concentrically wrinkled and striated epithccal 

 plate (fig. A). From the centre of this the corallites 

 radiate in all directions, those in the middle of the coral- 

 lum taking a nearly vertical course, while those near the 

 margins are horizontal, or even deflexed, so as to run nearly 

 parallel with the epitheca, corallites holding an intermediate 

 position between these two points being bent at corre- 

 spondingly intermediate angles. The corallum is made up of 

 comparatively large-sized corallites interspersed Avitli smaller 

 '' coenenchymal" tubules; and the structure of these two 

 elements is very different internally. 



The corallites, as shown by microscopic sections, have a 

 unique internal structure. In longitudinal sections {i,e. in 

 sections parallel with their long axes) they are seen to be 

 divided into a central tube and an exterior tabulate area 

 (fig. D). The central tube is either completely uninter- 

 rupted, or is occasionally traversed by a delicate transverse 

 tabula (perhaps at intervals of a line or less), marking a tem- 

 porary arrest in the process of growth. In some cases the 

 central tube is filled Avith crystalline calcite ; but it is often 

 more or less completely occupied by an infiUing of the sur- 

 rounding matrix ; and there are commonly little accumulations 

 of the same material above each of the transverse tabulae just 

 alluded to. The central tube is surrounded by a zone of 

 tabulge, which spring from the wall of the corallite, and are 

 then bent downwards so as to become parallel to the long axis 

 of the corallite, finally joining tlie tabula next below. There 

 is thus formed a series of large circumferential vesicles, the 

 convexities of which are directed upwards and towards the 

 centre of the corallite. When divided fairly through the 

 centre, the internal structure is seen to be as above ; but when 

 the section is excentric it passes through the exterior tabulate 

 zone, and the cut edges of the vesicular tabulae appear as 

 cross lines, looking like ordinary tabulffi. As most of the 

 corallites are curved, an ordinary longitudinal section divides 

 them partly through the centre and partly exceutrically, so 

 that part of the tube shows the one set of appearances and 

 part the other. 



