DAVIDSON — PAL^ONTOLOQIOAL NOTES ON THE BRAOHIOPODA. 99 
seen, and have been correctly represented and explained, those on the 
interior surface of the smaller or dorsal valve are rarely discernible. 
The scars left by the adductor, or anterior occlusor, muscle were 
defined by Bouchard in Athyris concentrica ; but the impressions of 
the posterior pair could not be distinguished upon any of the 
numerous valves in his possession ; although it is most probable, not 
to make use of the word " certain," that in this, as well as all other 
species of the genus, the adductor or occlusor had a quadruple 
attachment to the ventral valve. 
Dr. Sandberger has represented the muscular impressions of both 
valves of Athyris or Spirigera undata ; but not in quite so precise a 
manner as could be desired, or as is really seen on some silicified 
internal casts of A. ambigua from the carboniferous limestones of 
Bakewell (Derbyshire), in the Museum of Practical Geology, and of 
which two enlarged illustrations are here appended (Lign. 2, figs. 1 
and 2). 
I have also ascer- 
tained that the spiral 
processes, and their in- 
termediate connecting 
lamellae are in Athyris 
ambigua similarly dis- 
posed to those of A. 
pectinifera, of which we 
have already given an 
illustration in our pre- 
vious paper, so that 
Lign. 3. — Interior of the Dorsal Valve of ^«/j!/r!s uwiJipua. 
there can exist scarcely Restored from specimens in the Museum of Practical Geology. 
any doubt that the 
same arrangement was common to all the species of the genus. 
Mindful of the popular character of The Geologist, it will be 
diducteurs " of Gratiolet) ; while the accessory divaricators are better known by 
the name of " accessory cardinals" or " cardinalis " of Owen. 
The occlusor is the " adductor " of the generality of authors ; the anterior occlusor 
being the " adductor longus anticus" of Owen ; the posterior occlusor, the " adductor 
longus posticus " of the same distinguished anatomist. 
The pedicle-muscles have been termed adjustors by Mr. Hancock, under the 
belief that they move the shell upon its peduncle, and adjust it ; while the 
" capsularis " of Owen, is Hancock's peduncular muscle. 
H 2 
