204 BRITISH CARBONIFEROUS BRACHIOPODA. 



(ventral), the right muscle, causing a traction upon the opposite side of the inferior 

 valve, makes it deviate a little to the right by a sliding, the extent of which I 

 should not know, a priori, how to measure. The double-cross muscle of the left 

 side, acting symmetrically, makes it deviate a little to the left. 



4. Muscles obliques postero-anterieurs (anterior occlusor) et antero-posterieurs 



(central and external adjustors). 



a. Simultaneous action. — They draw the valves together energetically. 



b. Alternative action. — Supposing always the superior (ventral) valve as a fixed point 



the " muscles postero-anterieurs acting from behind forward upon the inferior 

 (dorsal) valve, make it slide backward. The muscles " antero-posterieurs" acting 

 from the front backward upon the inferior valve, make it slide forward. 

 N.B. — If one admitted an alternative possible between the longitudinal oblique muscle 

 of the right side and the left side, their movements would evidently come in aid 

 of those of the cross muscles. 



5. Muscles tedonculaires (peduncular muscle) et muscles marginaux. 



These muscles leave also their traces upon the shell. The first erect (adjust) the body 

 upon the peduncle, and that in two ways — the first by a direct action, in the second place 

 by causing the fluids which fill the internal cavity of the peduncle to ebb into the body. 

 The second act exclusively upon the border of the great pallial lobes. 



It will therefore be seen, from what has been stated, that, although anatomists agree 

 as to the shape and position of the muscles, they entertain different views respecting some 

 of their functions. Thus, Mr. Hancock objects entirely to the notion of the sliding of the 

 valves in different directions over each other by the aid of the adjustors (protractor sliding 

 muscles of Woodward 1 ), a theory first propounded by Cuvier and Owen ; while Dr. 

 Gratiolet believes that the cross disposition of certain muscles, whether from behind 

 forward or whether from right to left, would lead one to imagine a compensated antago- 

 nism from which equilibrium would result during the simultaneous contraction of all the 

 elements; and that the oblique muscles transversely crossed of Cuvier, his "muscles 

 obliques postero-anterieurs" and " antero-posterieurs," were employed in the sliding 

 action of the valves. Mr. Hancock, on the other hand, observes that in Crania, when; 

 the muscular system is arranged after the plan of Lingula, there exists no sliding move- 

 ment, and that Mr. Lucas Barrett, who has seen Crania alive, has distinctly stated that 

 ''the valves open by moving upon the straight side, as on a hinge, without sliding of the 

 valves ;" but it would be out of place and presumptuous were I to dwell any longer upon 



1 According to Mr. S. P. Woodward, Lingula would possess a pedicle muscle; three adductor muscles, 

 the posterior pair combined; two pairs of retractors, the posterior pair unsymmetrieal, one of them divid- 

 ing; and two posterior sliding muscles. 



