80 INTRODUCTION. 



Animal unknown ; shell somewhat trigonal, transverse or elongated, inequivalve, with 

 or without a mesial fold and sinus ; hinge line shorter or longer than the width of the 

 shell ; cardinal angles obtusely rounded, or extended into wing-shaped expansions ; 

 external surface smooth, or variously costated; shell structure unpunctated; valves 

 articulating by means of teeth and sockets ; beak straight or recurved ; area in ven- 

 tral valve large or narrow, and nearly parallel sided, bent backwards, flat or concave, 

 and divided by a triangular fissure, always more or less closed by a pseudo-deltidium 

 notched in the vicinity of the cardinal edge ; area in the dorsal valve narrow, often linear, 

 and likewise divided by a wide open fissure, partly or entirely occupied by the cardinal 

 muscular process or boss. 



In the interior of the smaller valve the labial appendages were supported by calcified 

 lamellae, assuming the shape of two large conical spires, which nearly fill the interior of 

 the shell ; the ends of the spires are directed outwards, towards the cardinal angles ; the 

 bases of the hollow conical spires nearly meet at the hinge side, but are wide apart in 

 front ; the hinge-plate is divided into two portions, to which the crura of the spires are 

 attached ; the crura are united, at some distance from the hinge, by an oral lamella ; the 

 divisions of the hinge-plate are hollowed, perhaps for the insertion of pedicle muscles . 

 the cardinal muscles (adductor brevis of Owen), were no doubt fixed to the cardinal 

 process or boss, which is small, and situated in the notch of the hinge-plate ; under it are 

 seen four large elongated scars left by the adductor.^ In the interior of the larger valve 

 a short hinge tooth is situated on either side at the base of the fissure, supported by ver- 

 tical shelly plates, which extend from the beak to the bottom of the valve, thus forming 

 the fissure-walls and assuming various forms and directions in difi"erent species ; they are 

 either small, regularly diverging, or converging to diverge again, and extending to a 

 greater or less distance into the interior of the valve ; between these a great portion of the 

 free space at the bottom of the shell is filled up by muscles, which are generally divided 

 by a blunt longitudinal crest ; the adductor commonly producing a small longitudinal and 

 mesial oval scar, and on either side of which are situated the cardinal muscles ; pedicle 

 impressions cannot be detected in this valve. 



Obs. The shells placed under this head, have been sub-divided into many sections and 

 genera, often established upon unimportant or superficial variations, partaking more of 

 the specific than generic character. When the interior of a larger number of species 

 shall have been carefully examined, it is possible that the genus may require to be sub- 

 divided into a few minor sub-sections, but I do not consider our present knowledge of the 

 species sufficient to warrant the adoption of any other sub-sections than, perhaps, those of 

 Spiriferina, D'Orb., and Cyrtia, Dalman ; and even to these, I do not attach great value. 



' On a remarkable cast o{ Spirifer alatus, figured by Professor King, in hie work on 'English Permian 

 Fossils,' the quadruple adductor scars appear to be situated in pairs one above the other, in a somewhat 

 different manner to what we observe in the generality of Spirifers, as, for example, Sp, striatus, Lonsdalii, 

 &c. On the same specimen, the vascular impressions are well preserved. 



