84 INTRODUCTION. 



presence of a mesial septum, which may perhaps entitle those and similar species to be 

 placed together as a small sub- section subordinate to Spirifer, as judiciously proposed 

 by Professor M'Coy.i It now remains to be ascertained whether all the species classed 

 under Cyrtia really belong to the sub-section,^ thus, for instance, the Sp. heteroclitus, 

 Demarlii, and a few others, which exteriorly appear to resemble the type of Cyrtia, 

 possess entirely dissimilar shell structure, which Dr. Carpenter has proved to be largely punc- 

 tated ; and these last would, therefore, have quite as valid a claim to be separated from 

 Cyrtia, as Spiriferina was, by its structure and plates, from Spirifer proper. It is 

 therefore evident that much more will require to be known of the internal and structural 

 character of the numerous sjjecies of Spirifer before the ye7ius can be properly and 

 definitely subdivided ; and all we can say at present is, that a portion of the species com- 

 posing the great genus Spirifer is punctated, while, perhaps, the greater number are 

 not so, and it will be interesting to ascertain, by a minute investigation of the known 

 species, how far the differences in the shell structure are accompanied by internal modifi- 

 cations. 



Geol. range. — The exact period at which Cyrtia first appeared is uncertain ; the types 

 of the sub-section lived during the upper Silurian era, and the last representatives, at 

 present known, are stated to have existed during the Trias. 



Examples : C. trapezoidalis and exporrecta, Dalman, Murchisoni, De Kon. -^ 

 cuspidata, Martin, sp. ; Calceola, Klipstein, sp. ; &c. 



Genus-— ki^^m^, M'Coy, IS 52." 



Type — A. tumida, Dal. or hekculea, Barrande. Int., PI. VI, figs. 71 — 7^. 



Atrypa (part) of Dalman, King, and some other Authors. 

 Athyris (part) M'Coij, 1844. 



Terebhatula, Sow. (part) 1815, and of the generality of Authors. 

 Spirigera (part) D'Orb., 1847. 



Animal unknown ; shell of a variable shape, circular, elongated, or transverse ; valves 

 more or less unequally convex, with or without a mesial fold and sinus ; beak apparently 



1 British Pal. Fossils in the Camb. Mns., 1852. 



'^ In the 'Tableau du Prodrome,' vol. iii, p. 56, we perceive that M. d'Orbigny has placed under 

 Cyrtia the following species: C. calceola, cristata, Schl. ; clorsata, M'Coy; hispanica, D'Orb. ; 

 laminosa, mesogonia, subconica, exporrecta, heteroclita, and trapezoidalis ; but we feel surprised that the 

 author should have admitted such an assemblage, since he positively states every time he has had 

 occasion to mention Cyrtia, that the shell structure was fibrous and impunctate ; 5. cristata, Schl., 

 in particular has been known some years to belong to M. d'Orbigny's section Spiriferina. 



^ This species is found in Belgium, and has lately been obtained from China, by Mr. Hanbury, and 

 presented to the British Museum. 



< Brit. Pal. Foss., 1852. 



