GASTROPODA. é 815 
Introduction.] 
dicular to the surface and mostly at right angles to the external lines of growth. 
The lamelle of the inner layer, which is commonly the strongest of the three, are 
also perpendicular but their direction is opposite. Tertiary Gastropoda often retain 
the microscopic structure of the shell ina very satisfactory manner, but as a rule the 
minute details are obscure, when not entirely obliterated, in fossil species. 
The form of the shell varies greatly, yet within the limits of genera and species 
it is remarkably constant. Three types are distinguished,—the tabular, of straight 
or only slightly curved form and occurring only among the Scaphopoda and Ptero- 
poda; the symmetrical, in which the shell is either conical or patelliform (Archina- 
cella) or involute (Bellerophon); and the spiral. 
The last is by far the most common type, and may in fact be looked upon as the 
typical form of the shell in the Gastropoda. In it the shell is essentially a spirally 
wound, elongated, conical tube, the coils or “whorls” being in most cases in contact 
and tightly cemented or amalgamated where they join. Sometimes the whorls 
are coiled nearly in the same plane, when the shell is said to be “discoidal,’”’ as” 
in many of the Huomphalide. More generally, however, the whorls are wound 
about the axis in an oblique manner, a true spiral being formed, the shell becoming 
“turreted,” “trochoid,” “turbinated,” etc. Occasionally the last whorl rises above 
the first or apical portion of the spire (Ophileta, Maclurea), but usually the embry- 
onic shell or “nucleus” is at the top of a cone formed by the gradually enlarging 
and descending whorls, the mouth or “aperture” occurring at the extremity of the 
last and largest whorl, termed the “body-whorl.” The whorls above the last 
constitute the “spire” of the shell. The line or groove marking the junction of the 
whorls is called the “suture.” A shell is said to be “imperforate” when the axis 
or “columella” is solid (Fusispira), or it is “perforated” when the axis is hollow 
(Trochonema), the axial cavity itself being known as the “umbilicus.” The “peris- 
tome” refers to the margin of the aperture; it is composed of an outer and an inner 
(or columellar) lip, of which the former is often expanded (Bucanospira, Salpingos- 
toma) or fringed with spines. The peristome may be continuous or “entire” or it 
may be interrupted or “incomplete,” in the latter case the left side of the aperture 
being formed only by the body-whorl. Not infrequently the aperture is drawn out 
and notched below, or there may be two notches, the second being above near the 
suture. These serve to protect the respiratory siphons. The posterior (upper) notch 
is probably represented by the median slit and perforation in Scissurella and Fissu- 
rella, and the same perhaps is true of the slit or notch in the outer lips of Bellerophon 
and Pleurotomaria. 
In most spiral shells the whorls normally are wound to the right, the aperture 
when in view beingon the right hand. In others, as for instance the recent Physa 
