
No. 432.] STUDIES OF GASTROPODA. 919 
such definite line of demarcation exists. In general, the proto- 
conch coils in the same direction as the conch, though con- 
Spicuous exceptions to this rule are found in Pyramidellidae 
and other groups. 
The early whorls of the protoconch of 
gastropods (except such ovoviviparous 
types as Cymbium) are smooth rounded 
coils of the type found in adult Natica, 
and best exemplified by Lunatia heros of 
our northern coast. In the majority of 
cases the initial whorl is minute, while 
the succeeding ones enlarge gradually and 
regularly. In some types the initial whorl 
is large and swollen (Fusus, Hemifusus, 
Fulgur, etc., Figs. 1, 7), when it is gener- 
ally more or less elevated and oblique, a early conch whorls. 
feature carried to excess in certain volutes. 
This type of protoconch has been termed “ bulbous” by Dall. 
Again the initial whorl may be compressed so as to produce 
a conical form, the sloping sides of which are flat. This type, 
observable in so-called Clavilithes from the 
British and American Eocene and in some 
other genera, may be denominated a “ tro- 
choid" type of protoconch.? The flatten- 
ing may be confined to the apical whorl 
or may be continued over one or more of 
the succeeding whorls. The normal nati- 
coid type of whorl may continue, with 
regular increase, thus giving rise to the 
large ** Melo " type of protoconch (Dall) so 
"s i prominent in Melo and some other genera. 
G. 3. — Protoconch of Syco- 
typus canaliculatus. pena When the whorls become depressed so as 
| epar um, umbi- practically to lie in one plane, a “ planor- 
boid” protoconch is produced (Dall). 
Again the naticoid initial whorl may be succeeded by a number 



1 Trans. Wagner aiu Znst., vol. iii (1890), p P. 
? 'The * trochiform " type of protoconch of ae is more comprehensive, includ- 
ing the naticoid type. 
