208 DR. F. J. NORTH 0>" [vol. lxxvi, 



Of these two forms, the second is undoubtedly the Zaphrentis- 

 Zone mutation of the species here described as T. laminosa, but the 

 identity of the first is not so clear. To illustrate it, Yaughan 

 quoted the figure of a specimen from Horizon y at Burrington 

 Combe, 1 and describes it as having fewer ribs than the second 

 form. The cardinal extremities of the figured specimen are broken, 

 and this, as has already been mentioned, conveys the impression 

 that the shell possesses fewer ribs than it actually does. The 

 figure so closely resembles an exfoliated Z 2 specimen as to suggest 

 that Vaughan's two forms are time-variants of the same species. 



VI. Spiriferina D'Orbigny. 



Shells referred to Spiriferina occur in Carboniferous, Permian, 

 and Liassic deposits in the British area ; and elsewhere in the Trias, 

 where that formation includes marine deposits. It has, in fact, 

 been customary to regard all Spiriferoid brachiopoda in which a 

 ventral median septum coexisted, or was supposed to coexist with 

 a punctate shell-structure, as Spiriferina, merely because there 

 was no other genus to which they could conveniently be referred, 

 and not from any failure to recognize the polyphyletic nature of 

 the species involved. 



The genotype of Spiriferina is a Liassic shell {Sp. rostrata 

 Schlotheim), and before we attempt to discuss the Palaeozoic 

 forms, some account of the genus in its original sense is necessary. 



The genus Spiriferina, was established by D'Orbigny, in 1847, 2 

 for certain Liassic brachiopoda previously included in Spirifer, but 

 which differed from typical members of that genus in the possession 

 of a vertical shelly septum in the ventral valve, and in the existence 

 of abundant relatively-large canals or punctse in the shell-structure. 



It is somewhat unfortunate that P/Orbign} 1- took as his geno- 

 type Sp. rostrata Schlotheim, because in external appearance that 

 species is not typical of the genus. It has a uniformly rounded, or 

 even globose shell, the surface of which may be quite smooth, or at 

 the most only slightly undulated ; while in the other species there 

 is usually a radial ornament consisting of well-defined angular costse. 



External characters: — Shape and dimensions.— The shell 

 is ovate, and the hinge-line less than the greatest diameter of the 

 shell. The cardinal extremities are rounded, and the lateral slopes 

 of each valve tumid. The size is variable, but the dimensions of 

 an average specimen are — width along hinge-line, 28 mm. ; length 

 of brachial valve, 21 mm. 



Usually, as in Sp. ivalcotti, the ornament consists of a few promi- 

 nent angular costse separated by V-shaped furrows, on each side 

 of a median fold in the brachial valve and a corresponding sinus in 

 the pedicle-valve. The fold and sinus ai*e more or less angular and 

 resemble the costae and furrows, from which they differ only in size. 



1 Vaughan [28] pi. xxxi, fig. 8. 



2 C. E. Acad. Sci. Paris, vol. xxv (1S47) p. 268. 



