[307] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 15 
Cape Cod. This shell is the Acmaa testudinalis, (Plate XXIV, figs. 
159, 159a ;) it is extremely variable in color, but is most commonly radi- 
ated, checked, or tesselated with brown, pale greenish, and white. It 
grows much larger on the coast of Maine than here. A peculiar narrow 
form of this shell, (var. alveus,) represented by fig.159d, lives on the leaves 
of eel-grass.. Beneath the rocks, and generally attached to their under 
sides, among hydroids, bryozoa, &c., several species of small, slender, 
pointed, and generally whitish shells occur, which belong to the genus 
Odostomia. The most common of these are O. trifida, (Plate XXIV, 
fig. 145,) O. bisuturalis, (Plate XXIV, fig. 146,) and O. fusca, (Plate 
XXIV, fig. 144,) but other similar species are often to be found. These 
all have the singular habit of spinning a thread of mucus by means of 
which they can suspend themselves from any surface. In confinement 
they will often creep along the surface of the water, using the bottom of 
the foot as a float, in a manner similar to that of many fresh-water 
shells. On the under sides of rocks are occasionally found some very 
beautiful and interesting naked mollusks; but this group of animals is 
far less abundant in this region than farther north. The largest and 
finest species observed here is the Doris bifida, (Plate XXV, fig. 176,) 
which grows to be about an inchlong. Its body is deep purple, specked 
with white and bright yellow, and the beautiful wreath of gills is cov- 
ered with bright golden specks ; the ends of the tentacles are also bright 
yellow. Its eggs are contained in convoluted gelatinous ribbons, which 
are attached to the under sides of rocks or in crevices. Another rare 
and curious species, the Doridella obscura, (Plate xxv, fig. 173,) is ocea- 
sionally found on the.under side of stones. This is a small, oval, flat- 
tened species, of a dark brows or blackish color, with small, white re- 
tractile tentacles on the back, but the gills are very small and situated 
underneath, near the posterior end of the body, in the groove between 
the mantle and foot. The eggs are inclosed in a delicate gelatinous 
string, which is coiled up something like a watch-spring, and attached 
to the under side of stones. 
Of bivalve shells several species are common on rocky shores, espe- 
cially in the crevices and under the rocks. Three kinds of muscles are 
usually met with. The species which lives at high-water mark, clus- 
tering about the small upper pools and in the crevices, and having its 
shell ribbed with radiating ridges and furrows, is the Modiola plicatula, 
(Plate XX XI, fig. 238.) This species is far more abundant, however, 
along the borders of estuaries and on salt marshes and muddy shores, 
always preferring the upper zone, where it is covered for a very short 
time by the tide. The most common species among the rocks, toward 
low-water mark, and in the larger pools, is the Mytilus edulis, (Plate 
XXXI, fig. 254,) which is the “common muscle” all along our coast 
from North Carolina to the Arctic Ocean. It is perfectly identical with 
the common muscle of Europe, which there forms a very important ar- 
ticle of food, and in many places, as on the coast of France, is exten- 
