DAVIDSON — ON SOME FOSSIL BRACIJIOPODA. 



413 



divided by a triangular fissure, open in the fry, but becoming gradually 

 more or less contracted with age, from the production and development 

 of one or two curved plates, notched in the vicinity of the cardinal edge, 

 and to which the term pseudo-deltidium has been applied. In the 

 smaller valve there exists also a narrow hinge-area. The valves of 

 Spirifera, and of all the genera and sub-genera of this family, articulate 

 by the means of curved teeth developed on either side, at the base of the 

 fissure, and fitting into corresponding sockets in the opposite or smaller 

 valve. In the larger valve, the teeth are supported by vertical shelly 

 plates, which, after having formed the walls of the fissure, extend from 

 the beak to the bottom of the valve ; they are small or large, regularlj'- 

 diverging, or converging to diverge again, and extend to a greater or 

 lesser distance into the interior of the valve. Between, and at times 

 beyond these plates, the larger portion of the bottom of the shell is 

 filled up by muscular impressions, sometimes divided by a blunt 

 longitudinal crest. The adductor or occlusor muscle, which is destined 

 to the function of closing the shell, leaves a narrow mesial longitudinal 

 and oval-shaped scar, and on either side are situated the cardinal or 

 divaricator muscles, the antagonists of the occlusors, and the ofiice of 

 which was to serve in the opening of the shell.'^' In the interior of the 

 smaller or dorsal valve there exists two large conical spiral coils which 

 nearly fill the interior of the shell, the ends being directed outwardly 

 towards the cardinal angles, while the basis of the hollow conical spires 

 nearly meet at the hinge-side, but are v/ide apart in front. The hinge- 

 area is divided into two portions, to which the principal stems of the 

 spire are attached, and in the notch under the extremity of the umbonal 

 beak there exists a calcareous projection or cardinal process, to which 

 was attached the other extremity of the cardinal or divaricator muscle. A 

 little lower down on the bottom of the valve, are seen four large elongated 

 impressions left by the adductor— the anterior or posterior occlusor 

 muscle of Hancock. Therefore, although we have not been able to 

 discern clearly in the larger valve of Spirifer, those impressions formed 

 by the accessory divaricator, ventral pedicle, or adjuster muscle, nor that 

 of the dorsal pedicle or adjuster in the smaller valve, it is nevertheless 

 probable that these muscles did exist, but have not left such defined 



For ample details upon this subject the reader is referred to Mr. A. Hancock's 

 admirable Memoir " On the Anatomy of the Brachiopoda," recently published in 

 the Transactions of the Eoyal Society. 



