GENUS SPIRIFERA. Igy 



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 costse. 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. octo- 

 costaia* (Pal. N. Y. Vol. 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 re]5arded the presence of a septum dividing the muscular area as characteristic 

 of Spieiferina. In the example cited above, the shell is silicified, and it is not easy to decide positively 

 its texture, but it appears to be fibrous. Should it prove to be punctate and a Spiripkrina, it would 

 carry this genus back to Silurian times. 



