816 THE PALEONTOLOGY OP MINNESOTA. 



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and the Lower Silurian Clisiospira, the volutions proceed in the opposite direction 

 with such constancy as to be eminently characteristic. The apical part of the 

 shell, which is directed backwards in all except some of the Patellidce, presents 

 important characters, as it contains the nucleus, or part formed in the egg, and 

 the primative whorls, which are often very different from the succeeding turns. 

 Careful investigation of the apex is likely to throw very reliable light upon the 

 evolution of the fossil types. 



In a large proportion of the Gastropoda the posterior portion of the foot secretes 

 a calcareous, horny, or fibrous plate called the "operculum," which serves to 

 close the aperture. Its inner surface is marked by a muscular scar whose lines 

 bear no relation to its external lines of growth, nor is its form like the muscular 

 scar in the shell. It begins its development in the embryo, the point from which 

 it commences its growth being called the nucleus. Further growth may take place 

 around the nucleus in a concentric or spiral manner, or the nucleus will be marginal 

 the additions occurring on one side only. The spiral forms may make only one 

 or two turns, or there may be as many as twenty. Opercula that were fit for 

 preservation as fossils seem to be comparatively rare in Paleozoic rocks. When 

 they are found it is nearly always as separate pieces so that it is in most cases 

 very difficult to decide to which of the associated shells they really belong. Still, 

 some of the early Paleozoic forms are interesting, being of unusual types, that 

 of Maclurea especially being remarkable for the strong internal process to which 

 the muscle was attached. 



Regarding the markings of the external surface of the shell, the "lines of 

 growth," which are more or less fine lines or stronger plications running parallel 

 with the edge of the apertural lips, are nearly always distinguishable. Their 

 importance to the paleontologist, who only too often must be satisfied with imper- 

 fect specimens, is obvious. Sculpture and color markings are longitudinal or 

 vertical when they take the direction of the axis, and revolving when they follow 

 the spiral. Of the more common varieties of sculpture we may mention the 

 striate, in which the surface is covered with fine lines either longitudinal or revolv- 

 ing; the carinate, when the revolving sculpture is prominent and sharp; the plicate 

 referring to a vertically ribbed sculpture; the cancellate, in which fine and straight 

 vertical and revolving lines cross each other nearly at right angles; the reticulate, 

 in which the decussating lines are not straight; the punctate, granulose, nodose and 

 spinous, referring to conditions sufficiently expressed by the terms applied to them. 



Finally, as regards the application of measurements, the distance between the 

 apex and basal extremity of a gastropod shell is termed the hight, while the 

 width or diameter of a spiral shell is the distance through the body-whorl at its 



