480 THE PALEON'TOLOGY OF MINNESOTA. 
[Preservation and methods of study. 
Cardinal teeth: Refers to the teeth situated on the hinge in the region of the 
beaks. 
Lateral teeth: One or more, generally elongate, subhorizontal teeth or interlocking 
ridges, often situated at the posterior extremity of the hinge.’ 
Muscular impressions: That of the anterior adductor, when present, is situated near 
the margin in the antero-cardinal region. It may be as large or much 
smaller than the posterior adductor, which, when both are present, is placed 
at some point in the postero-cardinal region. When only one adductor scar > 
is present (Monomyaria), or the anterior one is much the smaller of the two 
(Heteromyaria), the posterior scar is situated nearer the center of the valve. 
Umbonal scars are small impressions in the umbonal cavity, while the pedal 
muscles often leave small scars above and behind the anterior adductor 
impressions. 
Pallial line: This is a more or less sharply defined line running nearly parallel with 
the free margins of the valves and connecting the two adductor scars. Among 
paleozoic representatives of the class the line is usually simple, but among more 
recent forms a sinuate pallial line (said of it when its posterior part is bent 
more or less strongly inward), is quite common. 
Clavicle: A thin plate or ridge in each valve, of varying length, extending from the 
hinge margin, immediately in front of the beaks, vertically downward, or 
curving slightly forward. Example, Clidophorus. 
PRESERVATION AND Merruops or Srupy. 
In common with the Gastropoda, and probably for the same reasons, the paleo- 
zoic Lamellibranchiata are oftenest found in the condition of casts of the interior. 
This is true, especially of specimens preserved in dolomitic limestones like those of 
the lower Trenton and Galena in Minnesota, Wisconsin and Illinois, and the Niagara 
of northern Illinois and Wisconsin. These dolomitic specimens are to be regarded 
as in a favorable state of preservation so far as study is concerned. The shell, 
though dissolved away, has left good moulds of both the exterior and interior in the 
matrix, so that with the aid of plastic gutta percha the student is enabled to produce 
counterfeits of the shell that for purposes of classfication are scarcely to be excelled. 
To make good impressions it is often necessary to clean the moulds of the small 
crystals and other foreign matter that may in part occupy the space originally filled 
by the shell. Unfortunately, collectors too often are careless in preserving the 
outer mould, believing it, perhaps, of little consequence. In the interests of paleon- 
tology I would recommend greater caution and a lessening of the number of frag- 
ments by an early application of the contents of the glue pot. 
