438 Mr. T. Davidson on the Brachiopoda. 



ones, each of which is divided into two bundles or branches after 

 leaving its points of attachment, towards the middle of the shell. 

 They unite the valves by direct means, and we find moreover 

 some muscles which intersect or cross each other, but these 

 last have rather for function the movement of the portions of the 

 body than those of the valves. The perfectly dried condition of 

 the muscles generally permits of these being distinguished in 

 specimens preserved in collections, and are so disposed that we 

 are enabled to follow their mechanism with the eye. This 

 mode of opening constitutes for the Brachiopoda a most suitable 

 and proper distinguishing character, and renders needless any 

 other more complicated definition for them. 



"If nature has thus meant to furnish the Terebratula with 

 a mechanism so suitable, then Cuvier^s opinion that Lingula 

 opened its valves by means of these fleshy oral arms, using 

 them as a wedge^ is no longer tenable. If things had so 

 operated, the arms would constantly find themselves in a con- 

 strained position from the compression of the valves, and would 

 become inconvenient organs. It seems to us much more evi- 

 dent, that both valves in the animal [Lingula) must not bear the 

 same denomination, since the dorsal [ventral) valve possesses (as 

 in Terebratula) a longer beak, and its sides present equally a 

 kind of auricular expansion, so that these are situated low 

 enough to allow that the ventral side may separate from the 

 dorsal one on the principles of a lever. 



" Cuvier also expressly points out that we find at the summit 

 of the hinge a muscle which unites the valves in the most direct 

 manner : it is probably the opening muscle. On the contrary^ 

 the two muscles which unite the valves towards the middle of 

 their length have for function the closing of the valves. 



" It is probable that in Crania and Orbicula [Discina) the 

 mechanism is more difficult to explain : however, it is certain that 

 it must be analogous, since those animals possess so many 

 analogies. ^^ 



This evidence is so clear, that no comment will be required to 

 demonstrate that it is to Prof. Quenstedt the merit devolves of 

 having first discriminated and published what Prof. M'Cog now 

 claims — no doubt from not having been acquainted with the 

 paper of which we have given a translation ; but had even these 

 observations, published in 1835, not existed, I still maintain that 

 the claims of the author of the ^ British Palaeozoic Fossils ' could 

 not be maintained. 



I have added in Plate X. fig. 5. a reduced copy of the im- 

 proved diagram illustrating the muscular system in Terebratula 

 by Mr. Hancock, to help those who are not so familiar with the 



