214 CALYPTR^lD^. 



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 egg-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 grooA'^es. Occurs on Asteria. 

 G. astericola. Ad. and Reeve, 



BROCCHIA, Bronn. Irregularly conical, apex slightly spiral ; 

 left margin with a profound sinus ; posterior half of the margin 

 folded. 2 sp. Tertiary. A doubtful group. G. sinuosa, Bronn 

 (Ixiv, 80). 



Platyceras, Conrad. 



Syn. — Acroculia, Phillips. 



Distr. — Possil,50sp. Silurian to Carboniferous ; United States, 

 Europe. P. ventricosum, Conr. (Ixiv, 81, 82). 



Shell depressed subgiobose, 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 strongl}^ lamellose 

 transversely, nodose or spiniferous. 



The subgiobose species resemble the Velutinse, 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 (Ixiv, 83). 



IGOCERAS, Hall. Shell straight, with cancellated surface and 

 often with the addition of longitudinal plications. Silurian. 

 P. pileatum, Conrad. 



Berthelinia, Crosse. 



Bistr. — 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. 



