GENUS SPIRIFERA. 
coalesce, and again diverge, extending into the interior of the valve and 
partially surrounding the muscular area. It often happens that the 
pseudo-deltidium constitutes the exterior indication of the filling of the 
rostral cavity; and in old individuals, at least, this portion of the shell is 
quite solid. The occlusor muscular impressions are usually marked upon 
the inner surface of the shell, and upon the cast, by a mesial longitudinal 
oval scar; while the divaricator muscular markings occupy a wider 
space on each side, which is often deeply and beautifully striated. The 
general interior surface, beyond the muscular impressions, is usually 
nearly smooth, or marked by the impressions of the external costae. It 
not unfrequently ‘occurs, however, that the papillose surfaces, indicating 
the ovarian spaces, are strongly marked. 
There is usually a slight median crest separating the muscular impres¬ 
sions; and in some species this becomes a distinct septum, reaching from 
the base of the muscular area to the apex of the fissure, as seen in S. odo- 
costata* (Pal. N. Y. Yol. iii, p. 205). 
In the interior of the dorsal valve we have the distinct dental fossets 
just without the deltoid fissure, which is limited by the bases of the 
crura proceeding from the cardinal muscular process. The crura, converg¬ 
ing at a short distance from their bases, are bent backwards into the cavity 
of the valve, and again recurved, making several turns, until they are 
produced into two large conical spires which- nearly fill the interior of 
the shell, the apices being directed towards the cardinal angles. The 
double occlusor muscular markings are often well preserved in the cavity 
of the valve and upon the cast of the interior. The impressions of the 
divaricator muscles are marked upon the median cardinal process, or, in 
the absence of a distinct process, upon the apex of the notch or upon 
either side (See Plate 36). 
Besides the forms referred to the Genus Spirifera proper, there are at 
least three modifications of the type which are recognised by Mr. David- 
* Mr. Davidson has regarded the presence of a septum dividing the muscular area as characteristic 
of Spiriferina. In the example cited above, the shell is silicified, and it is not easy to decide positively 
its texture, hut it appears to he fibrous. Should it prove to he punctate and a Spiriferina, it would 
carry this genus hack to Silurian times. 
