6oO SAMPSON. [Vol. XI. 



Chiton, and projecting at the sides under one another, we 

 should have a rude representation of the condition of the shells. 

 The median dorsal muscle I regard as probably an extensor, for 

 the median dorsal line is obviously increased in length when 

 the animal is contracted, and conversely diminished when the 

 animal is extended. The muscle is attached from anterior to 

 anterior edge of the shells, so that it is impossible that the 

 muscle by its contraction should draw two shells at an angle to 

 each other and itself form the third side of the triangle, as 

 might occur if the muscle went from the anterior edge of one 

 shell to the posterior edge of the next posterior shell (from the 

 tip of the finger to the second joint). It seems probable that 

 the oblique dorsal muscles likewise function as extensors. 

 The lateral longitudinal muscles, on the other hand, are on a 

 line that is diminished when the animal is contracted, for the 

 maximum overlapping of the lateral edges of the shells occurs 

 at that time; these muscles, then, are to be regarded as con- 

 tractors. 



It is reasonable to suppose that these last two muscles, and 

 also the muscles of the cushion, are used in motions that in- 

 volve the two sides of the animal in different ways, and it is 

 possible that this is the chief function of the oblique dorsal. 



The muscles of the cushion in the sagittal and horizontal 

 planes (cz and c^ probably serve to readjust the shells at the 

 extension of the animal. The median dorsal tends to draw 

 the anterior shell over the posterior, and does not control the 

 posterior edge of the shell. The sagittal muscles {c^) of the 

 cushion, acting on either side of the median line, and the 

 horizontal muscles (^3) controlling the shells more laterally 

 where they are curved, tend, by virtue of their oblique posi- 

 tion, to draw the posterior shell from under the anterior, and 

 at the same time to draw the surfaces of the shells together; 

 or, in other words, these muscles lie in the direction of the 

 resultant of a force parallel with the surfaces of the shells, 

 and of a force perpendicular to the shells. The muscles in 

 the transverse plane at the antero-lateral edge of the cushion, 

 perhaps serve to draw the antero-lateral edge of the apophysis 

 nearer to the dorsal shell on contraction of the animal, a motion 



