GENERA OF FAVOSITID^E. 125 



am, however, quite satisfied that this view is untenable, and 

 that the condition of the septa is too variable to allow of its 

 being used as a generic character. A. suborbicularis, Lam., 

 seems often to possess such a single septal ridge in the interior 

 of each corallite (see fig. 20, and PI. VI., fig. 2) ; but an exami- 

 nation of a number of well-preserved specimens from the Eifel 

 has convinced me that it is excessively difficult to detect this 

 feature even in the most perfect examples, and that it is seem- 

 ingly really not invariably present, since it is only occasionally 

 that such a structure can be made out by thin sections, so far as 

 I have seen. It may be freely conceded, however, that certain 

 species have the septa reduced to one or three longitudinal 

 ridges, though no more than a specific value can be assigned 

 to this. On the other hand, A. Labechei, E. and H., though 

 stated by Edwards and Haime to possess a " slightly devel- 

 oped" or "very indistinct" septal ridge, like that of A. subor- 

 bicularis, can be conclusively shown by microscopic sections to 

 possess numerous vertical rows of septal spines (PI. VI., figs. 

 3 and 3 a) precisely similar in their characters to those of F. 

 Hisingeri, E. and H., and other species of Favosites. The same 

 feature is equally observable in some other species. In certain 

 types, again, such as A. Goldfussi, Bill. (PI. VI., fig. 4), the 

 septa appear to be entirely obsolete, no traces of their existence 

 being observed in thin slices. Upon the whole, then, we must 

 regard the development of the septa in Alveolites as quite 

 variable, and as affording characters of no more than specific 

 importance. 



Lastly, the tabulae in Alveolites are always well developed, 

 and are complete and essentially horizontal. These structures, 

 however, present no features of special importance. 



The known species of Alveolites, as here defined, are confined 

 to the Upper Silurian and Devonian periods ; and the two 

 species which I shall select for brief description as illustrating 

 more fully the characters of the genus are A. sub orbicular is, 

 Lam., and A. Labechei, E. and H. 



