480 THE PALEONTOLOGY 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, Clidqphorus. 



Preservation and Methods op Study. 



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. 



