96 MONOGRAPH OF THE GENUS GNATHODON—DALL. vol.xvii. 



Gnathodou seems to be indifferent as to the salinity of the water in 

 which it lives, as it is found both in the sea outside of the lagoons and 

 in the brackish water of the lagoons, while the living specimens received 

 by me from Mobile Bay seemed to maintain perfect health for some 

 four or live days in perfectly pure fresh water. But there is no doubt 

 that it is, by preference, like the oyster,' an inhabitant of waters the 

 salinity of which has been diluted by their proximity to the mouths of 

 rivers or creeks. In common with the majority of pelecypods inhabit- 

 ing fresh or brackish water, it has acquired the habit of secreting a 

 very heavy shell which is almost always eroded a good deal by the free 

 carbon dioxide of such locations. 



The T)eculiar hooked or jaw-shaped anterior lateral, which, in connec- 

 tion witli the longer posterior lateral, is the most marked characteristic 

 of the genus, results from the inequality of the two laminte between 

 which it is inserted in the opposite valve. In 31uUnia (from which Gnath- 

 odon seems to be an off-shoot) the lamiuic and teeth are alike short and 

 somewhat removed fromthe vicinity of the cardinal teeth. In Gnathodon, 

 however, the lamime are prolonged until they are very close to the car- 

 tilage pit behind and to the cardinal teeth in front. The lower anterior 

 lamina, for some unknown reason, did not attain the same length and 

 there is a gap between the cardinal tooth and theendof thelower lamina. 

 In Gnathodon, as in other pelecypods, the surface of the mantle is 

 produced in such a way as to secrete aud deposit the shelly matter 

 demanded by the growth of the hinge. The ventral exposures of the 

 hinge and its laminte are those upon which deposition is most profuse 

 and direct, consequently the gap referred to was raj^idly filled bydei)o- 

 sition from below on the ventral face of the projecting part of the 

 ui)per lamina. The process may be seen in its successive stages iu 

 any good series of Gnathodon cuneatus. Once the "hook" is formed, 

 it molds to a greater or less extent the form of the tooth impinging 

 upon it, and is preserved, among other reasons, because the triangular 

 buttress which it finally becomes is the most efficient obstacle which 

 the hinge possesses to the rotation of the valves on the cartilage as a 

 center. The tendency to this rotation, potentially very injurious, has 

 been promoted by the degeneration and immersion of the ligament. 

 Consequently it is not at all improbable that the "hook" is a character 

 which would be enlarged and preserved by natural selection. The 

 oldest species [clathrodon) has it least developed, the most abundant 

 recent species [cuneatns) most so. It is distinctly present in all the 

 known species, but not always conspicuous. In the adult the efficient 

 action of the hinge is promoted by distinct, usually transverse, crenu- 

 lations on opposed surfaces. Where the surfaces are flat the crenula- 

 tions are usually parallel grooves, but on rounded surfaces, such as the 

 point of the lower anterior lamina in the right valve, they may be 

 wavy, granular, or irregular. The end they serve is that of decreas- 

 ing the tendency to any wobbling of the hinge, and these crenulse ar^ 



