464 



THE GEOLOGIST. 



cular impressions do not appear to have been satisfactorily explained , 

 there are four principal scars visible on the bottom of the shell, which 

 Dr. Sandberger has attributed to the adductor, but which, if we are to 

 interpret them in accordance to what we find in other genera, we would 

 require to refer the two smaller ones on each side of the mesial ridge to 

 the adductor or occlusor muscle, and the larger one on either side of 

 these to the cardinal muscle or divaricators of Hancock, which last 

 would have their other extremity fixed to the cardinal process of the 

 opposite valve ; and thus we would have clearly defined, in their proper 

 position, the adductor which officiates in the closing of the valve, and its 

 antagonist, the cardinal or divaricators, the function of which was to 

 serve in the opening of the valves. No trace of pedicle or adjuster muscles 

 has been detected in the interior of either valve. Some further researches 

 will also be required in order to determine the exact point of attach- 

 ment of the spiral appendages. 



The genus Davidsonia was established by Mr. Bouchard, for the 

 reception of certain Leptsena-like shells, but which adhered to marine 

 bodies by a larger or smaller portion of the surface of their ventral 

 valve. The shells are transversely oval, with thick unequal valves, a 

 straight hinge-line area and fissure, which last is entirely covered by a 

 convex pseudo-deltidium. In the interior of the ventral or attached 

 valve, between and below the dental projections, are situated the 

 muscular scars left by the adductor or occlusor muscle, and on either 

 side of these are visible the larger impressions produced by the cardinal 

 or divaricator muscles. In the smaller valve between the largely- 

 developed socket-walls and close to the hinge-line, a small cardinal 

 process is observable, which served for the attachment of the cardinal 

 or divaricator muscles, and under this, on the bottom of the valve, is 

 seen the quadruple impression of the adductor or anterior and posterior 

 occlusors, so that we have the two sets of muscles for the opening and 

 closing of the valves complete. 



No trace of adjuster muscles could be ascertained, and these 

 may or may not have existed in those unperforated species which 

 were provided with a straight hinge-line, but I have observed 

 imprints of adjuster muscles in those Strophomena- shaped species 

 which were provided with a foraminal aperture for the passage of a 

 peduncle, as in Strophomena miahga, &c. The most important character 

 of Davidsonia resides in the greater portion of the interior of the ventral 



