[383] INVERTEBRATE AFIMALS OF VINEYARD SOUND, ETC. 89 
dant species is generally the Astyris lunata, (p. 306, Plate XXI, fig. 110,) 
which generally occurs among the small algve and especially on the 
Tubularians, in countless numbers; Anachis avara (p. 306, Plate X XT, 
fig. 109) is often found in considerable number ; bittium nigrum (p. 305, 
Plate XXIV, fig. 154) and Triforis nigrocinctus (p. 305, Plate X XLV, fig, 
152) are usually common and the former often is very abundant ; Cerith- 
iopsis Greenii (Plate XXIV, fig. 153) sometimes occurs, but is rare; Ilya- 
nassa obsoleta (p. 354, Plate XXI, fig. 113) and Tritia trivittata (p. 354, 
Plate XXI, fig. 112) are common, especially the former; Urosalpine 
cinerea (p. 306, Plate XX, tig. 116) is generally to be found at or below 
low-water mark on the piles and buoys; Bela plicata (Plate XX, fig. 
107) is sometimes met with, but is not common ; Odostomia bisuturalis 
(p. 307, Plate XXIV, fig. 146) and other species of the genus are often 
found near low-water mark on the piles, especially where they are 
somewhat deeayed. Littorina palliata (p. 305, Plate XXIV, fig. 138) 
and ZL. rudis (p. 305, Plate XXIV, fig. 137) nearly always occur near 
high-water mark, on the piles, where there are algie. In the harbors, 
where the water is brackislf, and less frequently in the purer waters, 
the Alexia myosotis (Plate X XV, fig. 168) may be found on timbers and 
piles near high-water mark, and sometimes, also, Skenea planorbis, (Plate 
XXIV, fig. 142,) Littorinella minuta, (Plate XXIV, fig. 140,) and Rissoa 
aculeus, (p. 806, Plate XXIV, fig. 141.) Among and feeding upon the 
Tubularians growing on the piles at and just below low-water mark, the 
beautiful Molidia pilata (Plate X XV, fig. 174) may often be found, espe- 
cially in the harbors where the water is more or less brackish. 
Another related species, apparently the Cavolina gymnota, was found 
by Professor Todd, on an old wreck in the Wood’s Hole passage, but it 
(differs in several points from any form that has been described. The 
branchiz were arranged in six transverse simple rows, on each side, 
those of the second and third longest; in the anterior rows there were 
four to six branchie, the lower ones much shorter than the upper ones. 
In life the branchiz were dark green or blackish. 
Several other Gastropods are occasionally met with in these situa- 
tions, but the species above named are about all that ordinarily oceur. 
Among the Lamellibranchs, or “ bivalve-shells,” we find the Teredo 
tribe, nearly all of which are peculiar to submerged wood-work, either 
fixed or floating, and most of them are capable of doing great damage, 
both to ships and to the timber and piles of wharves and bridges, or 
other similar structures. Although popularly known as the * ship- 
worm,” these creatures are not at all related to the worms, but are true 
mollusks, quite nearly allied, in many respects, to the common *long- 
clam” (Mya) and to the Pholas. Like those shells the Teredo excavates 
its holes or burrows merely for its own protection, and not for food; but 
the Teredo selects wood in which to form its holes, and when these have 
been excavated it lines them with a tube of shelly material. The holes 
are very small at the surface of the wood, where they were formed by 
