214 CALYPTRAHIDA. 
anteriorly thin and strap-shaped, posteriorly thick, orbicular 
and concave. 
These animals are said to feed on the sea-weed that grows 
around them, and on small marine organisms. They appear to 
have but limited locomotion, being usually adherent and modi- 
fying the margin of the aperture of the shell according to the 
surface on which they live Sometimes they wear away the 
surface beneath their foot, forming shallow excavations, or they 
secrete an imperfect shelly base by means of the same organ. 
The ego-cases are membranous and are attached in a tuft at the 
front of the foot under the neck. 
tHycA, H. and A. Adams, 1854. Shell conical, transparent, 
slightly curved, with longitudinal grooves. Occurs on Asteria. 
C. astericola, Ad. and Reeve. 
BRoccHIA, Bronn. Irregularly conical, apex slightly spiral ; 
left margin with a profound sinus; posterior half of the margin 
folded. 2sp. Tertiary. A doubtful group. C. sinuosa, Bronn 
(xiv, 80). 
PLatyceraAs, Conrad. 
Syn.—Acroculia, Phillips. 
Distr.—F ossil,50sp. Silurian to Carboniferous; United States, 
Europe. PP. ventricosum, Conr. (Ixiv, 81, 82). 
Shell depressed subglobose, subovoid or obliquely subconical ; 
spire small; volutions few, sometimes free and sometimes con- 
tiguous, without columella; aperture more or less expanded, 
often campanulate, and sometimes with the lip reflexed ; peris- 
tome entire or sinuous. Surface striated or cancellated, often 
spirally ridged or plicate, and -sometimes strongly lamellose 
transversely, nodose or spiniferous. 
The subglobose species resemble the Velutine, but there is 
every degree of variation in form between these and non-spiral 
shells. From among these, two subgeneric groups have been 
rather arbitrarily separated. 
orTHONYcHIA, Hall. Body of the shell straight or curving, 
gradually diminishing above, arched or in some degree spiral 
at the apex, with the last volution or more quite free. Sil. 
to Carb. P. spirale, Hall (lxiv, 83). : 
1GOcERAS, Hall. Shell straight, with cancellated surface and 
often with the addition of longitudinal plications. Silurian. 
P. pileatum, Conrad. 
BERTHELINIA, Crosse. 
Distr.—B. elegans, Crosse (Ixv, 94, 95). Fossil. Paris basin. 
Capuliform, very small, microscopic, thin, rather smooth, few- 
whorled, the spire very small and lateral, the last whorl greatly 
dilated with a large aperture. 
