SUB-GENUS PRASOPORA. 209 



They also become much reduced in number in the deeper parts 

 of tangential sections, when the large corallites become at the 

 same time more markedly polygonal, and more entirely united 

 by their walls. In any case, the small corallites are aggregated 

 at intervals to form clusters or "maculae," which have a star- 

 shaped form, and branch out between the larger tubes. 



Long sections (fig. 44, d) show a most remarkable structure 

 as regards the tabulae of the large corallites, these structures 

 beino- of two distinct kinds in the same tube. The tabulae 

 of one kind form a series of laro^e closed lenticular vesicles 

 generally on one side only of the tube, and usually not actually 

 in contact with one another. It is the cut edges of these 

 singular curved tabulae which give rise to the peculiar excen- 

 trically perforated plates seen in tangential sections of the 

 large corallites. In addition to these, there exist in the large 

 corallites perfectly straight horizontal tabulae, which are never 

 numerous, and which may pass completely from side to side 

 of the tube, or may, more commonly, merely extend from the 

 wall on one side to one of the convex closed tabulae on the 

 other side. The small corallites always have numerous com- 

 plete and horizontal tabulae. 



The type-form of M. Selwynii occurs in the Trenton Lime- 

 stone of Canada ; but there is found in the Cincinnati formation 

 of Ohio a form which possesses a very similar internal struc- 

 ture, and which I propose, in the meanwhile, to separate as a 

 distinct variety, under the name of J\I. Selwynii, van Jiospitalis, 

 Nich. In its habit and size, and in some of the minor points 

 of its organisation, this form differs considerably from the 

 typical examples from the Trenton Limestone, which I have 

 selected as the basis for the preceding specific diagnosis ; but as 

 it is clearly a very close ally of M. Schvynii, I prefer to regard 

 it at present as nothing more than a strongly marked variety. 



M. Schvynii, var. hospitalis, is invariably an attached form, 

 all the numerous examples which I have seen being fixed to 

 the exterior of the shells of Brachiopods. In form they are 

 hemispheric, rarely nearly globular, and their general size is 



o 



