DE. T. DAVIDSOX OX EECEXT BRACIIIOPODA. 



159 



muscles would bo fixed to the bottom of tlic valve, nearly iiiider the extremity of the touguc- 

 shaped "ascending shelly process" already descril)ed (Plate XXIII. fig. IG, cj), and be again 

 attached to the two thin shelly processes (Plate XXIII. fig. 15, b) which we have described 



Fkr. 10. 



Fisr. 17. 



V\<x. IS. 



After Lacazc-Diithiors, ' Annalcs des Sciences Xaturollcs,' Zool. t. xv. pi. i. figs. 7, 8, 9. 



Fig. K). Longitudinal section of TheckUum, to give an idea of the manner in which the median muscles act : n 

 curved lamelhr, which are supported by and attached to a small septum under the beak in the ventral 

 valve : h, median adductor muscle of Lacaze-Duthiers ( = occlusor of Hancock); c, adductor muscle of Lacaze- 

 Duthiers, which is attached by one extremity to tlu; bottom of the ventral valve, and by the other end to 

 the extremity of the quadrilateral cardinal process of the dorsal valve. 31. Laea/.e-Duthiers states that 

 the mechanism of the opening and occlusion of the shell can be thus formulated : when the animal 

 wishes to open its shell, the power, P, is represented by the muscle (c), its point of application being at 

 the extremity of the cardinal process of the ventral valve ; the resistance, E, is the weight of the valve 

 which has to be raised ; the point of support is found at A ; we have, thus represented, a lever of the 

 first kind. When the shell has to be closed, the same analysis would be applied to the lateral muscles, 

 reversing, however, the power and the resistance, the point of support being always at the articulation A. 

 Fig. 17. Animal, with the lobes of the mantle spread out, viewed from the side of the shell so as to exhibit M. 

 Lacaze-Duthiers's {a) muscles adducteiirs lateraux (Hancock's adjusters) ; J, Lacaze-Duthicrs's " muscles 

 abductours medians " Cllancock's occlusors), of which one extremity passes between the " muscles abduc- 

 teurs " (<■), and the other is directed outwardly to make room for the intestine ((7) which is the continua- 

 tion of the lobes of the liver (e) ; c is Hancock's divaricator. 



Fig. IS. Animal, separated from the shell, and seen on the face which corresponds to the coTicavo valve : a. lateral 

 adductor muscle of Lacaze-Duthiers ; h, median adductor muscle, of which the extremity only can be 

 seen, which attaches itself to the bottom of the lamcllai under the beak of the ventral valve : c, adductor 

 muscle slightly curved, of which the two extremities can be seen. 



as existing in the cavity of the beak. The function of these muscles (as stated by j\[. 

 Lacaze-Duthiers) would also be to effect a closing of the valves, but less efficaciously than 

 the preceding pair. The third pair, designated " muscles abducteurs " l)y the same author 

 (woodcuts, figs. IG, 17, 18, c), arc Mr. Hancock's " divaricators," and tlieir function Avoukl 

 be to open the valves. They form large impressions on tlie bottom of the ventral valve, on 



