816 THE PALEONTOLOGY OF MINNESOTA. 
[Introduction. 
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 Patellidw, 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 
