Lithodomus. MOLLUSCA. Pinnad^. 373 
wedge-shaped behind, and with slightly prominent 
of quartz and felspar, which sometimes occur in the 
beaks. The valves are covered externally with a 
hard limestone of the pillars, are untouched. Cuvier 
dark, thick, horny epidermis, and are pearly within. 
informs us that in their young state, the Lithodomi 
The species are boring shells, and are not numerous. 
suspend themselves to rocks by a byssus ; and that it 
They are found principally in the West Indies and 
is not till they are adult that they begin to perforate. 
New Zealand; but one well-known species is a native 
The cavities which they form in limestone rocks, corals. 
of the Mediterranean, and has often been quoted by 
shells, &c., are shaped like the shell, and do not admit 
geologists as proving the fact of a change of the level 
of free rotatory motion. 
in sea-coasts in modern times. In the Bay of Naples, 
LITHODOMUS DACTYLUS, the Mediterranean species. 
near to Pozzuolo, is situate the ruined temple of Jupiter 
is eaten by the natives residing on the coasts. It is 
Serapis. Onlj'' three pillars remain standing, and at the 
collected in quantities by the fishermen, and sold as a 
height of ten feet above the base each pillar exhi- 
luxury in the markets. It possesses a remarkable phos- 
bits a zone of six feet in height, where the marble has 
phorescent quality. “ The fishermen place the animal 
been perforated by these shells. The holes are four 
in the sun, and with it besmear their hands and faces at 
inches in depth ; and it is observed that the nodules 
night, so as to illuminate them as with phosphorus.” 
OEDER III.— 
PECTIN-ACEA. 
Family— TEIGONIIDZE. 
the particular arrangement of the hinge teeth vary con- 
siderably in the different species, and various genera 
The Trigonias have the shells thick, closed, and 
have accordingly been formed to receive them. 
trigonal or somewhat heart-shaped, with the beaks 
Genus Pectunculus. — This genus has an orbicu- 
directed posteriorly. The hinge is composed of two 
lar, nearly equilateral shell; the valves closed, smooth. 
diverging cardinal teeth in the right valve, transversely 
or radiateiy striated, and invested with a fimbriated 
deeply striated on both sides ; and four in the left, stri- 
epidermis. The beaks are separated by a grooved, 
ated on one side only. Externally the valves are 
lanceolate, ligamental area. The hinge is semicircular. 
ornamented with radiating tuberculated ribs, wdiile in- 
and the teeth are small and transverse. The species 
ternally the surface is beautifully iridescent and pearly. 
number about fifty, one half of which are natives oi 
The species are few in number, and are all from the 
America ; the others being found in Europe, India, and 
Australian seas. The genus is represented in Plate 
New Zealand. They inhabit muddy and sandy coasts, 
11, fig. 18, by Trigonia pectinata. 
ranging from eight to sixty, and, in some instanees, to 
a hundred and twenty fathoms. 
Family — ARC ADZE. 
Genus Nucula. — T his genus has a trigonal or 
obliquely ovate shell, with the beaks turned towards 
The Arks have a large foot, which often secretes a 
the short posterior side. The valves are smooth or 
laminated byssus — thus adapting the animal to anchor 
sculptured, and covered externally wdth a smooth olive 
itself. The shell is angular, elongated, or roundish, 
epidermis, while the interior is pearly, and has the 
sometimes closed, and sometimes gaping inferiorly. 
margins crenulated. The hinge has a prominent in- 
The hinge consists of a number of transverse, inter- 
ternal cartilage pit, and the teeth form a range of 
locking teeth, placed in a straight line, which appear 
comb-like denticles on each side. The species are 
to be formed b}' the subdivision of two elongate lateral 
rather numerous. They do not spin a bj'ssus, but use 
teeth. In general the cartilage is external, arising 
their foot for burrowing, and are able to erawl upon it 
from diverging angular lines marked upon the facet. 
like a Gasteropod ; indeed Professor Forbes tells us 
formed by the gradual thickening of the dorsal edges, 
that he has seen one of our native species creep up the 
which causes the umbones to be separated from each 
other as the shell enlarges. The byssus in this familjq 
side of a glass of sea-water. 
instead of being composed of silky threads is in form 
Family — PINNADZE. 
of a horny cone, composed of numerous thin plates, 
occasionally becoming solid and calcareous, and which 
The species of this farnily have a large foot which 
the animal can cast off and form again with great 
secretes a powerful and very fine silky byssus. The 
rapidity. 
shell is large, triangular, and of a prismatic texture. 
THE TYPICAL ARKS (represented in Plate 11, fig. 
The valves are thin in general, more or less fragile, 
19, by ArcaNoce) have the shell equivalve or nearly so. 
and gaping posteriorly; externally they are smooth. 
the valves covered with an epidermis, and strongly 
or scaljq or obliquely furrowed, and internally shining 
ribbed or cancellated externally ; the margins are 
and nacred. The beaks are terminal, the hinge straight. 
smooth or dentated, gaping or closed, and sinuated 
long, and toothless, and the ligament linear and internal. 
ventrally ; the hinge margin is straight, and the carti- 
About tliirt}^ species of Pinnae have been described, and 
lage is placed in small marginal pits. The species are 
they are found in nearly all seas, ranging from low 
numerous, and attach themselves to rocks, stones, &c.. 
water to sixty fathoms. They usually live buried in 
by means of their peculiar byssus, or conceal them- 
the sand, or between cracks in rocks, with their sharp- 
selves in holes and crevices. The general form and 
edged, gaping, truncated ends just above the surface. 
