DAVIDSON — ON SOME FOSSIL BRACHIOPODA. 
413 
divided by a triangular fissure, open in the fiy, 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, regularly 
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 office 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 wide apart in front. The hinge- 
area is divided into two portions, to v/hich 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 Royal Society. 
