72 
PECTUNCULUS costatus, 
TAB. XXVII. — Fig. 2. 
Spec. Char. Orbicular, depressed, with twenty- 
five longitudinal sharp ridges,, and a few trans- 
verse stride ; hinge of fourteen teeth ; margin 
serrated within. 
Shell thin, somewhat variable in form: some specimens 
are rather oblique and angular; the ribs are not quite 
regular, being in some more distant than in others, with 
here and there a small intermediate one. The beak is not 
prominent. A few specimens are found an inch or more 
in diameter, the one figured is of a middling size. 
Mrs. Tylee and the Rev. Mr. Bingley, have favoured 
me with variety of this from Hordwell Cliff. I wonder 
that Blander has not figured it, as it does not appear to 
be rare. 
PECTUNCULUS plumstediensis. 
TAB. XXVII.— Fig. 3. 
Spec. Char. Transversely obovate^ slightly oblique^ 
with longitudinal obscure furrows and minute 
transverse striae; margin serrated within. 
Beak rather prominent, obtuse; shell thin, nearly an inch 
wide. 
Found in the alluvial sandy gravel of a small hill which 
contains many good shells, at Plumstead, near Woolwich. 
I believe it is seldom found whole, being very brittle. I 
have fragments larger than the one figured, and some 
minute whole shells; this may occasionally be worth 
noticing, as it may lead us to improvement in geological 
science, and probably serve to shew in what cases the shells 
remain in their original beds. 
