ACM AA. 263 
branchial plume issues from the back of the neck ; in a state 
of quiescence it does not appear long, but can be greatly ex- 
tended and contracted, like a pair of ladies’ cross-barred 
forceps ; it is composed of a pectinated leaf of almost hori- 
zontal, compact, rather short, blunt strands: the artery and 
branchial vein are easily observed ; the plume gradually curves 
and tapers from its origin to a pointed termination, and is of 
the palest uniform hyaline drab ; it may be often seen exserted 
and vibrating like a tentaculum in all directions ; the anus is 
close under the plume, and emits large pale yellow cylin- 
drical pellets. I should have mentioned that the foot, when 
the young are expelled from the matrix, serves as a nidus for 
the membranous vesicular envelope, which when the embryos 
have left it is discharged, as in Janthina. 
It inhabits the coralline zone at Exmouth in abundance, 
adhering to old bivalves and other substances. 
This genus shows a considerable departure from Patella in 
the character and figure of the branchial leaf, and conducts us 
to Calyptrea and Pileopsis. 
We have not seen the animals of the three following species 
—with us, demee—A. testudinalis, A. fulva, and A. ancyloides, 
as they are not generally inhabitants of the southern coasts of 
Britain. Scotland has the greater claim to them. From what 
we have learnt of two of the animals, we prefer depositing 
them in Acmea, rather than adopting the new genera, “ Pili- 
dium” for the “ fulva,’ and “ Propilidium” for the “ ancy- 
loide”; and we will only further observe, that if the latter has 
two branchial plumes, it ought, agreeably to our system, to be 
referred to the first Patellidan group ; but the references will 
enable malacologists to judge for themselves. 
A. TESTUDINALIS, Miiller. 
A, testudinalis, Brit. Moll. ii. p. 434, pl. 62. f. 8,9; (animal) pl. A.A.f.2. 
A. rutva, Miiller et nobis. 
Pilidium fulvum, Brit. Moll. iu. p. 441, pl. 62. f. 6,7; (animal) pl. 
AVA, £. 3: 
A. aAncyLorpes, Forbes et nobis. 
Propilidium ancyloide, Brit. Moll. ui. p. 443, pl. 62. f£.3, 4,5; (ani- 
mal) pl. A. A. f. 4. 
