Mr. T. Davidson on the Brachiopoda. 437 



in his theoretical considerations, he is of opinion that the elas- 

 ticity of the calcareous appendages [of Terebratula] is sufficient to 

 enable the animal to raise the reflected portion [of the loop] so as 

 to force open the opposite valve, it must be remarked, that he 

 has himself demonstrated that in the greater number of cases 

 this explanation is not applicable. I believe, moreover, that 

 in the instances indicated, the calcareous appendages had no 

 other function than to protect the soft internal organs, and that 

 they can hardly furnish the proper means of opening the shell. 



" If we examine the non-perforated or ventral valve [dorsal of 

 Owen*) of any Terebratula whatever, we immediately remark that 

 the inner and incurved extremity of the beak [umbo) is entirely 

 hidden under the deltidium of the dorsal valve [ventral of Owen). 

 On opening the shell this extremity is deeply sunk (or hid) 

 under the deltidium, since the points of attachment on which 

 the ventral [dorsal) valve is moved are formed by the two lateral 

 teeth of the dorsal [ventral) valve ; these teeth are perfectly esta- 

 blished at the height of the auricular expansions, and correspond 

 with the sockets of the ventral [dorsal) valve. Since this point 

 of support is placed on each side towards the middle of the 

 beak, it follows, that a force applied to this beak must open the 

 valve as a lever would do. In effect we remark also at this ex- 

 tremity two well-defined muscular impressions, which lead us to 

 suppose the presence of muscles which must often have been 

 largely developed, so as to have transformed the extremity of 

 the beak into a large surface. These muscles direct themselves 

 from thence towards the large anterior impressions of the dorsal 

 [ventral) valve, and by the most direct road, without crossing 

 each other. These contractions must, therefore, have deter- 

 mined the opening of the shell , and it is with justice that they 

 have received the denomination of opening muscles [Oeffnungs- 

 muskeln). The double muscular impression situated on either 

 side of the dorsal line [septum) might be easily explained, inas- 

 much as each of the two opening muscles divided itself into two 

 bundles or branches in order to obtain a larger surface of 

 attachment. 



" The closing muscles, on the contrary, direct themselves from 

 the ventral [dorsal) valve under the point of support towards the 

 bottom of the beak, so as to produce the closing of the valves by 

 their contraction ; they also divide themselves each at its point 

 of attachment to the ventral [dorsal) valve into two branches. 

 TerebratulcB therefore possess two closing muscles and two opening 



* Prof. Quenstedt makes use of the old system of calling the perforated 

 valve the dorsal one, &c, I have placed within brackets Prof. Owen's (and 

 now generally admitted) mode of denominating the valves. 



