SUTURE, APERTURE, ETC., IN UNIVALVES. 
31 
and is usually medially placed between the 
suture and the base, following the spiral course 
of the volutions. In the carinate species, as 
TTelix lapldda, the keel 
decisively indicates its posi- 
Fic. 63. — Univalve 
with Carinate Periphery. 
Ifelix lapicida L., 
St. Vincent Rocks, Clifton 
near l>ristol, . . , , , , 
Coii.byMr.j.w.cunciaii. tioii, aiKi less sti'ongly SO 
Fig. 64. —Univalve with 
Sub-carinate Periphery. 
Helix caper at a Mont., 
Perth, 
Coll, by Mr. Hy. Coates. 
Fig. 65. — Univalve 
with Rounded Periphery. 
Helix neJfiofalis L., 
Isle of Lisniore, 
Coll, by Mr. A. Somerville, 
B.Sc., F.L.S. 
when the keel is only faintly developed, as 
in Jlelir cdperaUi, when the .shells are termed 
sub-carinate or angulated; 
but in those species with regnlarly convex 
whorls, as Helix nemoralis, its precise position 
is more difficidt to define. 
The Suture is the line of junction of one 
whorl with another, and varies in character and 
distinctness in accordance with the convex or 
planate outlines of the whorls. It may even 
be canaliculate or channelled, crenulated, or puckered, or simply 
more or less deeply impressed. 
The Aperture of the shell is the part last 
formed and is the opening through which the 
animal protrudes its body ; it may be almost 
exactly round, semi lunar, or other .shape, and is 
sometimes so greatly contracted with teeth or 
folds as to form almost a matter of surprise how 
the animal can insinuate its body through the 
constricted space. 
The Peristome or Peritreme is the margin of the apei'ture and 
may be distinctly continuous and detached during 
the whole life of the animal, as in Cydustoma 
eIe(j<tnK, or only distinctly continuous and de- 
tached at matur- 
ity, as in Helix 
hipicida and in 
the Clmmlicv. 
It may however 
have the con- 
tinuity of its outline tind character 
broken by the penultimate whorl. 
Fig. 66. — Univalve 
with a Contracted 
Aperture. 
antiveriigo (Dp.) 
X 20, 
Hemsworth, Yorks., 
Coll, by Mr.C.Ashford. 
Fig. 67. — Univalve 
with Detached and Con- 
tinuous Peristome. 
Cyclos. elegans (Midi.), 
Preston Candover, 
Hants., 
Collected by 
Mr. H. P. Fitzgerald. 
Figs. 68 & 69. — Helix lapicida L., 
Wells, Somersetshire, 
Showing the Continuous and Detached 
Peristome of Adult shell, and the Interrupted 
and Simple Peristome of the Immature stage. 
Collected by Rev. S. Spencer Pearce, M.A. 
