DAVIDSON — PAL^ONTOLOGICAL NOTES ON THE BRACHIOPODA. 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 concentrlca ; 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 umftif/Mfl. 

 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 Rmcock' s peduncula^^ muscle. 



H 2 



