No. 432.] STUDIES OF GASTROPODA. 927 
by the succession of characters in the shell whorls, from the 
earliest (nepionic) conch stage to the adult (ephebic) stage. 
The revolving elevated lines, or “ spirals,” may in general be 
considered as primary and post-primary. The primary ones 
are the first to appear, and they increase in number by the 
exogenous appearance of new ones on the upper and lower 
portions of the whorl, outside of those which appeared first. 
Secondary spirals appear between the primary ones as these 
diverge, owing to the uniform increase in size of the whorls. 
Tertiary spirals and spirals of a higher cycle appear in special- 
ized forms, but all such forms begin with only primary spirals, 
and generally only a few — sometimes even only one of these. 
The higher cycles come in later progressively, being most 
numerous in the adult stage. This teaches us that in the 
primitive type of a series we may expect to find primary spirals 
only, even in the adult, and experience shows that these are 
characteristic of the earlier members of any series, and that 
they generally accompany simple ribs and simple rounded 
whorls. (See Figs. 6 and 7.) 
The first modification in the form of the whorls in gastropod 
shells is generally a change from the primitive rounded outline 
to an angular one, which causes the division of the whorl by a 
median or submedian keel, or carina, into an upper, more or 
less flattened “ shoulder” portion and a lower, generally larger 
body portion (Fig. 5). Sometimes more than one carina 
arises, in which case all except the lower portion of the whorl 
becomes. flattened, the section assuming the appearance of a 
portion of a polygon. The angulation, which finally develops 
into a carina, and often into a more or less accentuated “ keel," 
is generally due to the strengthening of one of the primary 
spirals above the others. When the shell is ribless the angu- 
lation becomes a smooth keel. This feature appears early in 
the pleurotomarioid shells, in which the angulation is generally 
accompanied by an emargination or sinus of greater or less 
depth, which occupies the place of the carina. This group of 
shells, which appears to be a very heterogeneous one, proba- 
bly had its beginning in the Lower Cambrian species of 
Raphistoma, of which Æ. attleborensis Shaler and Foerste is 
