No. 432.] STUDIES OF GASTROPODA. 925 
not appearing visible until some time after. On the other hand, 
fine radiating lines — the precursors of the visible spirals — are 
seen in the embryonic hyaline shell of Fulgur, etc., before it 
has become opaque by the deposition of secondary calcareous 
material (Fig. 8). It is most probable, however, that these 
fine thread-like markings are 
merely due to a change in the 
texture of the hyaline shell, 
corresponding in that respect 
to the primitive lines of 
growth, and are not equivalent 
to the elevated spirals, whose 
existence is due to a regular, 
though slight, plication of the 
mantle edge which builds the 
shell If we adopt this view as 
the most probable one, the ribs, ' rs. iy hyaline shell of Fulgar (Syco- 
so far as my observations go, Wid fia h m 
must be considered the first 
modification of the shell in the ribbed and spiraled forms. 
It is, however, by no means true that spirals do not appear until 
after the ribs have been formed. In a great many Paleozoic 
genera ribs never occur, while spirals are well developed. This 
is well exemplified in the Ordovician genera Cyclonema and 
Trochonema, the most primitive types of which (ż.e., T. (Gyro- 
nema) livatum U. and S., etc.) are umbilicated shells of a round- 
whorled or naticoid outline, with spirals more or less strongly 
developed. Among the earliest ribbed shells of naticoid form 
is Holopea pyrene Billings from the Middle and Upper Trenton 
group of Canada and central United States. The ribs are scarcely 
anything more than coarse undulations, very like the dying 
stages of the ribbed condition in more highly specialized genera 
of later geological periods. They have, however, the chief 
qualities of the ribs in their most accentuated development, 
and must be regarded as such. As nearly as can be deter- 
mined from the illustrations given by Ulrich,! the earliest 
Stages are free from undulations, thus indicating that, as we 
1 Paleontology of Minnesota, vol. ii, Pl. LXXIX, Figs. 13-18. 

