64 
248. 
THE MOLLUSCA OF DORSETSHIRE. 
Class PELECYPODA. 
The classification of the Pelecypoda has been always a matter 
of great difficulty owing to their comparatively uniform 
structure. The muscular impressions were for a long time 
solely relied upon. An arrangement based on the existence 
of one or two adductor muscles only proved unsatisfactory, 
as well as the classification based on the form of the foot ; 
Fischer based his classification on the respiratory organs. 
That of Pelseneer, founded on the gill structures, is 
probably the best, and will be generally used. 
(Lamellibranchiata, Conchifera, Bivalves.) 
ARCIDA. 
BARBATIA, Gray, 1840. 
B. lactea (Z.) Arca lactea, LZ. 
Pulteney, p. 36, pl. 11, f. 5. 
Forbes and Hanley, Vol. ii., p. 238, pl. 46, f. 1-3. 
Secretes itself in the cavities of shells, and anchors itself to 
stones hy its byssus. 
North shore of Poole, Pulteney. 
Portland, dredged 15 fathoms, E£. Forbes, 
Weymouth, W. Thompson. 
Kimmeridge Bay, dredged, different depths, J. C. M.-P. 
Swanage, Miss Colson. 
Fossil: Coralline an Red Crags. 
PECTUNCULUS, Lamarck, 1799. 
. P. glycimeris (Z.) Area pilosa, Z. 
Pulteney, p. 36, pl. 11, f. 2. 
Generally distributed round the coast ; inhabits deep water. 
Weymouth and Swanage, Pulteney. 
Portland, West Bay, of a more ovate form, Z. P. Sykes. 
Kimmeridge Bay, dredged, 10 fathoms, J. C. M.-P. 
Dead valves thrown on shore between Studland and South 
Haven, J. C. M.-P. 
Fossil: Coralline and Red Crags, 
