ACM TEA. 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 j 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 lanthina. 



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 Calyptrcea and Pileopsis. 



AVe have not seen the animals of the three following species 

 with us, Acmaae 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 Acm&a, 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, MtQler. 



A. testudinalis, Brit. Moll. ii. p. 434, pi. 62. f. 8,9; (animal) pl.A.A.f.2. 

 A. FULVA, Miiller et nobis. 



Pilidiumfulvum, Brit. Moll. ii. p. 441, pi. 62. f. 6, 7 ; (animal) pi. 

 A. A. f.3. 



A. ANCYLOIDES, Forbes et nobis. 



Propilidium ancyloide, Brit. Moll. ii. p. 443, pi. 62. f. 3, 4, 5 ; (ani- 

 mal) pi. A. A. f.4. 



