MADREPORARIA PERFORATA. 313 



" mural pores." As here restricted, the family is exclusively Palaeo- 

 zoic in its range, the earliest known types appearing in the Ordo- 

 vician, while the latest are found in the Carboniferous rocks. The 

 great majority of forms, however, are Silurian and Devonian. 



There are certain appearances in thin sections of the Favositidce, as 

 seen under the microscope, which it is desirable to understand. In the 

 first place, the individual corallites of the colony have distinct walls. 

 Hence, in thin sections (fig. 194, A and c), the partition which separates 

 contiguous tubes usually exhibits a central dark or light line (the " prim- 

 ordial wall ") bounded on each side by a layer of fibrous " stereoplasma." 

 In badly preserved specimens it may not be possible to demonstrate the 

 primordial mural plate in thin sections, but the real distinctness of the 

 corallites is shown by the fact that in fractured examples the individual 

 tubes commonly separate from one another along the line of this plate, 

 each tube thus retaining its own proper wall. 



In the second place, the so-called " mural pores " of the Favositidtz are 

 rounded or oval apertures, usually arranged in longitudinal series, which 

 perforate the walls of adjacent corallites and place adjoining visceral 

 chambers in direct communication. They are sometimes not completed, 

 since the thin " primordial wall " which separates contiguous tubes is 

 sometimes not actually perforated. The mural pores are most regular in 

 fonn and distribution in the genus Favosites, in which the pores are often 

 surrounded by slightly raised margins, and are arranged in one or more 

 series along the flat faces of the prismatic corallites (fig. 195, D). In such 

 genera as Pachypora, where the corallites have greatly thickened walls, 

 the mural pores assume the character of tubes rather than of mere pores, 

 and in such cases they often perforate the walls in a tortuous manner. 

 As regards their recognition in thin sections, mural pores appear in 

 transverse sections of the tubes (fig. 194, A, B, c) as gaps or deficiencies 

 in the wall forming the circumference of the corallite, the size of this gap 

 depending on the size of the pores. In vertical sections (fig. 194, a', b', c') 

 the pores may similarly present themselves as gaps in the walls of the 

 corallites. It very commonly happens, however, that a vertical section 

 may in part rim along the actual wall of the corallites, instead of merely 

 dividing the tube longitudinally ; and when this happens, the mural pores 

 are seen as round or oval perforations in the wall within the space 

 included between the lateral boundaries of the tubes. 



The principal genus of the Favositidaz is Favosttes ( = Calamopora, 

 Goldfuss) itself, the species of which range from the Ordovician to 

 the Carboniferous inclusive. In this genus the corallum (fig. 195) 

 is commonly massive and often of large size, but in other cases it 

 may be lobate or branched. The corallites are numerous, usually 

 more or less conspicuously polygonal, with thin and distinct walls. 

 The mural pores are arranged in rows along the flat faces, or, more 

 rarely, along the angles, of the prismatic corallites. In the former 

 case, each face of the tube may carry one, two, or three rows of 

 pores, this being, generally, a constant feature in different species. 

 There are numerous tabulae, and these structures are usually " com- 

 plete " {i.e., they stretch entirely across the visceral chamber) ; but 



