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 Daiiclsonia was established by Mr. Bouchard, for the 
reception of certain Leptjeaa-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 impressiou 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. 
Ko 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 Stroplwmena analoga, &c. The most important character 
of Lacidsonia resides in the gicator portion of the interior of the ventral 
