18 - THE CHICAGO ACADEMY OF SCIENCES. 
The penis and penis-sac are each retracted by a powerful muscle 
which is inserted near the attachment of the right buccal retractor, 
the penis-sac retractor being the larger. (PI. IV, F, 3, 4; pl. I, figs. 
18, 19.) The penis-sac is protracted by two sets of muscles, one, 
anterior, being attached to the dorsal surface of the body and the other, 
posterior, to the columella muscle. The former set consists of a wide 
band of muscular tissue, made up of many smaller muscles} while the 
latter set may be composed of as many as six distinct muscles or as 
few as one muscle. (PI. I, figs. 15, 16.) There may also be two 
penis-sac retractors. The female organs have no free muscles. The 
retractors of the foot are attached to the columella muscle, as in most 
spiral shells. 
4. BIBLIOCRAPHY.! 
The following works have been consulted in the preparation of chapter I: 
Baker, F. C., 1900, b.; 1905, £—Baudelot, M., 1863.—Butterell, J. D., 1880.— 
Cooke, A. H., 1895.—Cuvier, G., 1816—Dybowski, W., 1884.—Eisig, H., 1869.— 
Keferstein, W., 1862-66.—Lacaze-Duthiers, Henri de, 1872.—Lankester, E. Ray, 
1891.—Lewis, James, 1870.—Moquin-Tandon, A., 1855.—Nelson, William, 1877.— 
Pilsbry, H. A., 1894, b.; 1900, b—Spengel, Dr. J. W., 1881.—Stearns, R. E. C,, 
1891, b.; 1901.—Sterki, Dr. V., 1893—Tryon, G. W., 1882—Wetherby, A. G., 
1879.—Whitfield, R. P., 1882.—Williams, J. W., 1889. 
1iSee the bibliography at the end of this volume, 
