GENERA OF FAVOSITID^E. 39 



series of types (e.g., F. Gothlandica, Lam.) the pores are bi- 

 serial, each prismatic face of the corallites carrying two rows, 

 usually placed alternately. In other forms, again, the pores 

 are triserial; while some species (e.g., F. alveolaris, Goldf., and 

 F. aspera, D'Orb.) are distinguished by the peculiarity that the 

 pores are situated in the angles formed by the prismatic walls 

 of the corallites, instead of on their flat faces. Very commonly 

 the pores are surrounded each by a raised rim or margin, but 

 this may be replaced by a circular pit, or the surface may be 

 quite plane. It should, lastly, be noticed in this connection, 

 that the mural pores are commonly very difficult of detection, 

 even in specimens otherwise perfectly well preserved, by even 

 the closest external examination. Very commonly in calca- 

 reous specimens, and sometimes in those which are silicified 

 (especially -in those where the silica has assumed the " orbicu- 

 lar " form), the walls of the corallites appear to be completely 

 imperforate under a hand-lens, or even under the microscope. 

 Thin sections, however, when taken parallel with the axis of 

 the tubes, will generally, in some part or another, coincide with 

 the plane of the wall of a corallite, and will then exhibit the 

 mural pores with greater or less distinctness. 



The calices of the typical species of Favosites are placed 

 parallel with, and not elevated above, the general surface ; but 

 in some forms which have not yet been clearly separated from 

 this genus, the calices open more or less obliquely to the surface, 

 and the lower lip of the calice is more or less elevated and pro- 

 jecting. In a few singular types (F. turbinata, Bill., F. For- 

 besi, var. tuberosa, Rom., F. clausus, Rom., &c.) there is the 

 singular feature that the calices in a larger or smaller number 

 of the corallites become closed by a calcareous lid or " opercu- 

 lum." Sometimes this seems to be merely the result of a con- 

 tinuous growth of the epitheca upwards ; but in other cases 

 the operculum would seem to be formed by successively 

 deposited concentric layers of calcareous matter, which spring 

 from the margins of the calice, and gradually close in towards 

 its centre. In the species above mentioned all of which are 



