TAVIDSON — SCOTTISH CARBONIFEROUS BRACHIOPODA. 
231 
h, AUenialh-c ff<-/<o«.— When the prcadductcurs contract themselves alone, 
they close the shell in front, and make it gape behind; when the post- 
adduetcurs contract ;ilonc, (hey close the shell almost completely behind, and 
make it open in front. 
2. — Muscles Peaussiers Verticaux [Posterior Parietal Muscles]. 
a, SimnUaneous act ion. — They depress the body behind, cause the internal 
fluid to flow towards the arras, and consequently come strongly in aid of the 
action of the " muscles post adducteurs." 
3. — Muscles Obliques Transversalement : Muscles Croises of Cuvier 
[Posterior Adtostors]. 
a, Siiiudtamom action. — They energetically draw together the valves. 
b, Alternative action. — Taking for a fixed point 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-A^'TERIEURS [anterior Occlusor] et 
Antero-posterieurs [Central and External ^\j)justors]. 
a, Simv.ltaneous action. — They draw the valves together energetically. 
b, Alternatice 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. 
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- 
gin aux. 
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 
agixe 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 slidmg of the valves in different directions over each other by the 
aid of the adjusters (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 obHque muscles transversely crossed 
of Cuvier, his " muscles obliques postero-anterieurs" and " antero-posterieurs" 
were employed in the sUding action of the valves. Mr. Hancock on the other 
hand 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, Lingula would possess a pedicle muscle ; thi-ee adduc- 
tor muscles, tie posterior pair combined ; two pairs of reti-actors, the posterior pair unsym- 
metrical, one of them dividing ; and two posterior sliding muscles. 
