DAVIDSON — SCOTTISH CARBONIFEROUS BRACHIOPODA. 



231 



h. Alter mtke ri'c^/o;^.— When the preadducteurs contract themselves alone, 

 they close the shell in front, and make it gape hehind; when the post- 

 adducteurs contract alone, they close the shell almost completely behind, and 

 make it open in front. 



2. — Muscles Peaussiers Verticaux [Posterior Parietal Muscles]. 



a. Simultaneous action. — They depress the body behind, canse^ the internal 

 fluid to flow towards the arms, and consequently come strongly in aid of the 

 action of the " muscles post adducteurs." 



3. — Muscles Obliques Transversalement : Muscles Crotses of Cuvier 



[Posterior Adjustors]. 



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



h, Alternative action. — Taking for a fixed poiiit the valve, which I call the 

 superior (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 ex- 

 tent 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. 



h. Alternative «^?//ci;«.— 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. 



c, N.B. — If one admitted an alternative possible between the longitudinal 

 oblique muscle of the right side and the left side, their movements would evi- 

 dently come in aid of those of the cross muscles. 



5. — Muscles Pedonculaires [Peduncular Muscle] et Muscles Mar- 



GINAUX. 



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*), 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 antagonism, 

 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 

 liand observes that in Crania, where the muscular system is arranged after the 

 plan of Lingula, there exists no sliding movement, and that Mr. Lucas Barrett, 



* According to Mr. S. P. Woodward, Liugula would possess a pedicle muscle; three adduc- 

 tor muscles, the posterior pair combined ; two pairs of retractors, the posterior pair vmsym- 

 metricaJ, one of them dividing ; and two posterior sliding muscles. 



