PLATE XII. 



Fig. I. Part of a vertical section of a young specimen of Plasmopora petaliformis, 

 E. and H., from the Wenlock Limestone of Dudley, enlarged five times, 

 showing the faintly-marked walls of the smaller corallites, and their convex 

 tabulae. 



Fig. 2. Portion of a tangential section of Heliolitcs inegastoma^ M'Coy, from the 

 Wenlock Limestone of Dudley, enlarged five times. 



Fig. 2 a. Part of a vertical section of the same specimen, enlarged five times, 

 showing the comparatively well-marked walls of the smaller corallites and 

 their strong horizontal tabulae. 



Fig. 3. Under surface of a small specimen of Pbiacopora Grayi, Nich. and 

 Eth. jun., from the Lower Silurian of Girvan, Ayrshire, of the natural size, 

 showing the concave base, with its concentrically-striated epitheca. 



Fig. 3 a. Upper surface of another specimen of the same, from which all the 

 short corallites except a few on one margin have been denuded, leaving the 

 superior aspect of the epithecal plate to view, of the natural size. 



Fig. 3 b. Portion of the corallum of the same, as naturally split along a hori- 

 zontal plane, showing the large and small corallites, with their visceral 

 chambers filled with dark matrix, — enlarged five times. 



Fig. 3 c. Part of a vertical section of the corallum of the same, embedded in 

 an opaque matrix, — enlarged five times. 



Fig. 3 d. Tangential section of another specimen of the same, infiltrated with 

 calcite, showing the large and small corallites (/, / and /, /),- — enlarged 

 twenty times. 



Fig. 3 e. Vertical section of another example of the same, enlarged twenty 

 times, showing the large corallites (/, /), and the smaller and more closely 

 tabulate tubes {t, t). [Figs. 3-3 e are copied from the " Monograph of the 

 Silurian Fossils of Girvan," by R. Etheridge, jun., and the author.] 



Fig. 4. Tangential section of a specimen of Chcetetes radia/is, Fischer, from the 

 Carboniferous Limestone of Russia, showing the thick and completely 

 confluent walls of the corallites, with an occasional inward projection in- 

 dicative of commencing fission, — enlarged ten times. 



Fig. 4 a. Vertical section of the same, enlarged ten times, showing the remote 

 and regularly placed tabulae. 



Fig. 4 b. Portion of surface of the same, enlarged ten times, showing the form 

 of the calices. 



Fig. 4 c. Portion of a tangential section of a specimen of the same, from the 

 Carboniferous Limestone of Shap, Westmorland, enlarged ten times. 



Fig. 4 ^. A small portion of the same section as the last, enlarged twenty 

 times, showing the complete amalgamation of the walls of the tubes, and 

 the presence in some of the tubes of the inward projections produced in 

 tlie process of fission of the old corallites. 



