COLUMELLAR MUSCLE AND OPERCULUM. 13 



the inner face, the small rounded pittings where they have been 

 attached. 



Dr. J. E. Gray was the first investigator who announced that 

 the operculum is homologous with the second valve of the lamelli- 

 branchiates or bivalve mollusks. He has shown that the oper- 

 culum is developed on the embryo long before it is hatched ; 

 that it is placed on and covers a particular part of the body 

 called the lobus operculigerus, and which bears to it the same 

 relation which the mantle does to the shell, and that its growth 

 occurs in the same manner ; that this growth is made by the ad- 

 dition of new matter to the inner surface and especially near 

 the margin ; that it is attached to the animal by means of one 

 or more muscles, which, as in the bivalve shell, pass from the 

 larger valve or shell to the smaller one or operculum ; that the 

 operculum, as it increases in size, is gradually moved on the end 

 of its muscle the many-whorled operculum of the Trochi re- 

 volves as many times on the end of the muscle as the many- 

 whorled spiral shell turns on its imaginary axis ; that the opercu- 

 lum is often lined internally with a shelly coat like a shell, and 

 sometimes, like the Cowries, its outer surface is covered also with 

 a shelly deposit by a special development of the opercular lobe. 



The principal difference between the operculum and the valve 

 or shell of the Gasteropods consists : 



1. In the operculum having no cavity, its cone being de- 

 pressed, flat or even concave, or very much compressed, form- 

 ing only a spiral riband, as in the spiral operculum. But this 

 absence of a cavity is a difference only of degree, for the valves 

 of some Gasteropoda, as Umbrella, Patella, etc., are much flat- 

 tened; the first resembling the annular operculum of Ampul- 

 laria and Paludina : but the greatest resemblance is to be ob- 

 served in the small, flat valves of Gryphyea, Exogyra, Chama, 

 and other genera of bivalve shells which are attached by one of 

 their valves. These valves are often quite as flat and destitute 

 of any cavity as the operculum of any Gasteropod ; and it is to 

 be remarked that these valves exactly resemble a spiral opercu- 

 lum in shape, the remains of the ligament forming a spiral mark 

 on the outer surface, showing how the valve has rotated on the 

 body of the animal as the operculum rotates on the foot of the 

 Gasteropoda. 



