Zool.— Vol. I.] BANCROFT— CHELYOSOMA PRODUCTUM. 32I 



which extend cleat- across the peripheral plates and are 

 united to the central ones. 



In our species every muscle, except those connected 

 with the siphons, is attached at some place on the disk, 

 extends nearly to the edge of the disk and then bends ven- 

 trally and is attached to the side wall of the test some 

 distance down. The inner fibres (the conspicuous ones in 

 fig. 11) are the longest, being attached farthest from the 

 edge of the disk and down the sides of the animal. Besides 

 the inner fibres there are a lot of shorter ones (m. s, fig. 

 11) that are attached nearer the edge of the disk at both 

 ends. The musculature is much better developed on the 

 left side, the fibres being longer and extending much 

 farther down the side. There is a tendency for all of these 

 muscles to be inserted near the centers of the plates, which 

 are indicated by the heavy black lines in the figure. But the 

 variation is so great that this tendency is hardly manifested 

 at all in some regions. It is to be noted, however, that no 

 muscles are ever inserted on the hinges. 



In the siphons the radial muscles are weaker and 

 situated internally to the circular ones. Centrally they are 

 attached near the lip of the orifice, and thence extend 

 beyond the region of the siphon, either losing themselves 

 among the peripheral muscles or joining these directly. 

 The circular ones consist of six series of strong fibres 

 extending from one siphonal ridge to the next. A fair 

 average condition of the fan-shaped muscles is seen in the 

 figure. They join the outer ends of the siphonal ridges 

 with the extra siphonal plates, and assist in closing the 

 orifices both directly by their contraction, and indirectly by 

 giving a more rigid support to the contracting circular 

 muscles. 



When we compare the two species of our genus, it is 

 found that the arrangement and variability of the plates 

 depend in great measure upon their relations with the 

 muscles. In C. macleayanum the plates appear to be very 

 constant in number and arrangement; for among the 

 thirty-four or more individuals of which accounts have been 



