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APPENDIX A. 
(LONSDALE ON CORALS.) 
secretions from the animal through foramina in the first instance, and horizontal channels subsequently : 
but in true Retepora the intermediate layer of vertical capillary tubes is totally wanting, the corals pos- 
sessing that structure, and formerly included in the genus, having been removed from it by Lamouroux 
and other authorities. Among the minor distinctions may be noticed, the oblique arrangement of the 
cells in Fenestella, whereby they in part overlie each other, and are more or less suddenly bent towards 
the oral termination ; also the tendency to a cylindrical form and vertically lineal arrangement. If the 
meshes in Retepora generally can be regarded as the result only of oblique unions of branches, then the 
transverse processes in Fenestella, whether cellular or not, may likewise be considered as a generic di- 
stinction. A central dividing ridge, or the sculpturing exhibited on the cellular surface of some species 
of Fenestella cannot alone he regarded as an essential character, though no analogous structure appears 
to have been noticed in Retepora, the strength of the configurations depending generally upon the age 
of the specimen or of the part preserved, and the very existence being not unfrequently concealed by 
external thickenings. There are other details which the practised eye will detect and dwell upon, and 
render available in the aggregate ; but it is hoped that sufficient have been indicated to prevent the most 
inexperienced paleontologist from assigning to Retepora such polypidoms as those under consideration, 
because they exhibit, at first sight, a certain amount of resemblance. 
Between Fenestella and Hornera the agreement in some respects again is greater than in either of the 
preceding instances, but there are important essential differences. In Hornera there exists each of the 
three great structural characters already shown to belong to Fenestella, namely, a polypiferous surface, 
an intermediate layer of capillary tubes, and a reverse outer crust produced at a certain period of deve- 
lopment, and thickened by secretions from the animal. In the cylindrical or slightly polygonal form of 
the polype cavity, as well as in the oblique overlying position, there are also certain but limited agree- 
ments, the cavity being of very limited extent in Fenestella. Hornera, however, has been shown by 
Dr. Milne Edwards to belong to the family of Tubuliporidae (Ilecherclies sur les Polypes, Mem. sur les 
Crisies, &c.), and it never exhibits the structures about to be noticed. In many published figures, a row 
of relatively large foramina are delineated ranging down the central ridge (where it exists) ; and they have 
been found to prevail in species possessing such divisional lines, whether one or more in number, but 
under different appearances, the foramina being sometimes near each other, sometimes at considerable 
distances, and generally in a straight row, though in one instance they were found to be arranged alter- 
nately or in zigzag. Occasionally they are entirely wanting. When the coral itself is not preserved, and 
the specimen exhibits only a cast of the surface, a conical indentation usually represents the foramen ; 
and the difference is conjectured to arise from the outer covering of the opening having been destroyed 
where the coral is in part preserved, but impressed in a perfect condition where the cast only is exhibited. 
In the cast of a fine large coral which exhibited all the structural characters of Fenestella?, but in which 
the polype-residences were disposed in quincunx without any divisional ridges, there were no lineally dis- 
posed foramina or indentations ; but over or a little on one side of many of the oral apertures, were 
minute, serai- spherical convex casts, and in a few instances a corresponding concave impression. Re- 
specting the true nature of these foramina or the chambers connected with them, no opinion is hazarded ; 
but it is believed that they are strictly analogous to the rows of apertures in the Lunulites, which have 
the cells arranged in parallel lines, and that the semi-spherical cavities in the specimen last alluded to 
may be considered identical in nature with the shallow cavities of frequent occurrence over the mouths 
in those species of Lunulites which have the cells disposed in quincunx. Beyond this comparison, and 
the inference that Fenestella should be considered as belonging to the same family of polypidoms as 
Lunulites, and therefore not to the Tubuliporidae, the author does not feel qualified at present to offer any 
remarks. 
