ACEPHATLA. 447 
are sculptured by radiating ramose ribs, bearing minute, pointed 
scales. Almost every shell, if closely examined, especially near the 
beaks, will exhibit some of these corrugations ; but others become for 
the most part smooth, lamellar and shining. Left valve emarginate 
near beak, leaving the upper point adunque; right valve flat, with a 
large semicircular byssus-fissure. Interior silvery white, with a very 
broad drab-white limbus: byssus-fissure a shallow sinus in right 
valve: opposed by a large, prominent, re-entering sinuous callosity : 
ligament grooves six or eight, delicate, short. 
Length two inches; height an inch and four-fifths ; breadth three- 
eighths of an inch. 
Inhabits Carlshoff Island, Paumotu Group. 
This shell, though so variable as to elude all terms of description, is 
still very distinct from all others, in all its forms. It seems to acquire 
its multiform shapes from the cavities in coral where it resides. 
Young or well-protected specimens are sculptured by delicate, ra- 
diating, ramose ribs, bearing minute scales. 
Figures 557, 557 a, outside and inside of the shell; 557 J, an elon- 
gated form ; 557 c, a young specimen. 
CRENATULA PERGAMINEA (Gould). 
T. membranacea, obliquissima, subfalcata, concentricé laminoso-fibrosa, 
albida, lineis lacteis radiata ; margine dorsal recto, apicibus tumt- 
dis, acutis, ad trientem anteriorem sitis ; latere antico arcuato et cum 
margine ventrali continuo ; latere postico oblique truncato : area car- 
dinalis angusta ad quinque crenata: intus submargaritacea. 
Crenatula pergaminea, GOULD ; Proceed. Boston Soc. Nat. Hist., iu. 
310. Nov. 1850. Expedition Shells, 91. 
SHELL thin, light, parchment-like, pale, pearly white, with fine ra- 
diating lines of milk-white, transverse, turning very obliquely back- 
ward in a foliate manner; hinge margin straight, the beaks, at the 
anterior third, acute, and the umbonal slope protuberant; posteriorly 
