INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 307 



Cape Cod. This shell is the Acmcm 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. alvens,) represented by fig. 159&, 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 0. trifida, (Plate XXIY, 

 fig. 145,) 0. bisuturalis, (Plate XXIY, fig. 146,) and 0. fusca, (Plate 

 XXIY, 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 XXY, fig. 176,) 

 which grows to be about an inch long. 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 occa- 

 sionally found on the under side of stones. This is a small, oval, flat- 

 tened species, of a dark browa 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 XXXI, 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 Mytiliis edulis, (Plate 

 XXXI, fig. 234,) 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- 



