CONCHIFERA. 425 
in crevices of rocks, and the empty burrows of boring mollusks ; 
they are often much worn and distorted. The genus Palearca 
probably belongs here; we have not been able to ascertain the 
generic characters; but they may be found in the Memoirs of 
the Geol. Sury., Canada, Vol. III., under the head Cyrtodonta. 
Distribution, 140 species. World wide, most abundant in warm 
seas; low water—230 fathoms (A. imbricata, Poli). Prince- 
Regent Inlet (A. glacialis). A scaphula, Benson, is found in 
the Ganges and its branches, from Calcutta to Humeerpoor on 
the Jumna, 1,000 miles from the sea. A second species has 
been found in the river Tenasserim, Birmah. The hinge is 
edentulous in the centre, and the posterior teeth are laminar 
and branched; the elements of the posterior muscular impres- 
sion are distinct. 
Fossil, 400 species. Lower Silurian—. United States, 
Hurope, South India. 
CucuLL@A, Lamarck. 
- Etymology, cucullus, a cowl. 
Type, C. concamerata, Pl. XVILI., Fig. 14. 
Shell sub-quadrate, ventricose ; valves close, striated ; hinge- 
teeth few and oblique, parallel with the hinge-line at each end ; 
posterior muscular impression bounded by an elevated ridge. 
Distribution, 2 species. Mauritius, Nicobar, China. 
Fossil, 240 species. Lower Silurian. North America, Pata- 
gonia, Europe. 
Sub-genus, Macrodon, Lycett. M. Hirsonensis, Pl. XVII., 
Fig. 15. Shell with a few oblique anterior teeth and one or 
more long laminar posterior teeth. The Ark-shells of the 
Palzeozoic and secondary strata have their anterior teeth more 
or less oblique, like Arca, the posterior teeth parallel with the 
hinge-line like Cucullea ; their valves are close or gaping below ; 
their umbones frequently sub-spiral; and the hinge-area is 
often very narrow, and in some species only the posterior moiety 
is yisible. 
Parallelopipedum, Klein. The outermost hinge-teeth short, 
and perpendicular to the hinge-line; teeth developed along 
the whole length of the hinge. 
PECTUNCULUS, Lam. 
Type, P. pectiniformis, Pl. XVII., Fig. 16. (Arca pectun- 
culus, L.) 
Shellorbicular, nearly equilateral, smooth or radiately striated ; 
umbones central, divided by a striated ligamental area; hinge 
