SPIRIFERID.E. 



101 



Sp. exjoorrecta ; but this is of little consequence, since both Dalman and Hisinger have 

 correctly described and figured Wahlenberg's Silurian Anomites exporredus, which we 

 must retain, omitting the reference to Walch. 



Spirifera (or Cpiia) exporrecta varies considerably in shape, or rather in the dimen- 

 sions and position of its area, as may be seen by a glance at the figures in om- Plate ; thus 

 the triangidar area, with an average breadth of eleven lines (at the hinge-line), and a 

 length of from three to seven lines from the point of the beak to the hinge-line, would 

 constitute Wahlenberg's, Hisinger's, and Dalman's Anomites exporrecfus; while Hisinger, 

 and after him Dalman, gave the name of trapezoidalis to those variations with a narrower 

 area, as seen in fig. 17 of our Plate. This is not, however, the only difierence observable 

 in the area of this remarkable species ; for in some examples it is completely flat, almost 

 equilateral and equiangular (fig. 16), and it recedes at a right angle, or even at more than 

 a right angle, from the plane of the dorsal valve, while in other specimens it is more or 

 less concave and incurved at the beak. Another character of great importance in this 

 species resides in the shape of its deltidium, which varies, it is true, slightly in different 

 specimens, but is always narrow and lanceolate; and, whatever may be its length, it does not 

 appear to much exceed one line and a half in width at its base. It also entirely covers 

 over the fissure ; and in the Swedish and British examples it presents an elongated 

 depression, which, commencing at the extremity of the angular beak, extends to a lesser 

 or greater distance along its surface, and even sometimes to within a small distance of its 

 basal extremity.^ At the inward extremity of this flat or slightly concave depression may 

 be seen in many specimens, bat not in all, a small oval aperture, which however is often 

 cicatrised, leaving only a slight mark to indicate its former presence. The animal was no 

 doubt attached during a part of its existence to marine objects by means of a pedicle, but 

 with this moorage it afterwards dispensed. These arrangements have been carefully 

 represented in the enlarged figures 19, 20, and 21 of our Plate.- This character of the 



1 Barrande's figures of the Bohemian specimens of this species show no indication of the depression 

 above described, nor of the foramen ; the deltidium being equally convex throughout. Hence, the depres- 

 sion in question may, perhaps, not always have been present. 



- For the reception of these shells, Dalman, in 1827, proposed the genus Cijrtia, with the following 

 diagnosis : 



"Testa inaequivalvis sequilatera, valvse majoris dorso in semiconum vel pyramidem dimidiatum 

 elevato, latere cardinali perpendiculariter piano, — foramine nuUo. Cardo rectilineus." But, as I have 

 already stated, at p. 83 of my " General Introduction," Dalman's diagnosis is unsatisfactory, and equally 

 applicable to several species of Spirifera ; and the only character of any value that I can perceive, by 

 which it might be distinguished from Spirifera, consists in its having the fissure entirely arched over 

 by a deltidium which was at some period of the animal's existence perforated by a circular foramen. Even 

 this last character, however, is not present in all the specimens referable to this so-termed genus. In one 

 remarkable form, Cyrtia Murchisoniana, the circular foramen, which sometimes presents a small, marginal 

 tubular prolongation, is situated at, or close to the extremity of the beak, or of a long narrow deltidium, 

 as may be seen in some Chinese specimens of the species which I described and figured in the ' Quarterly 

 Journal of the Geol. Soc' (vol. ix, p. SfiS, pi. xv, figs. 6 — 9, 1853). If, therefore, the term Cyrtia is to 



