~404 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [698] 
in the ancient Indian shell-heaps on the coast of Massachusetts, on the 
islands in Casco Bay, and at Damariscotta. The shells, in a semi-fossil 
state, have been dug up from deep beneath the mud in the harbor of 
Portland, Maine, in large quantities, but native oysters appear to be 
entirely extinct in Casco Bay. Very abundant in Long Island Sound ; 
in the upper part of Buzzard’s Bay; rare and local in Vineyard Sound ; 
very abundant on the shores of Maryland and Virginia. Mouth of 
Saint John’s River, and in Tampa Bay, Florida (Conrad). Texas 
(Roemer). 
Fossil in the Post-Pliocene at Point Shirley, Massachusetis, Nan- 
tucket Island (abundant), Gardiner’s Island; in the Pliocene of South 
Carolina; and in the Miocene of Virginia and South Carolina. 
The occurrence of large quantities of oyster-shells beneath the har- 
bor mud at Portland, associated with Venus mercenaria, Pecten irradians, 
Turbonilla interrupta, and other southern species, now extinct in that 
locality, and the occurrence of the first two species in the ancient In- 
dian shell-heaps, on some of the islands in Casco Bay, though not now 
found living among the islands, indicates that the temperature of those 
waters was higher at a former period than at present. These facts also 
point to the most satisfactory explanation of the existence of numerous 
southern shells, associated with the oyster and Venus mercenaria in the 
southern part of the Gulf of Saint Lawrence, though not now found in 
the intermediate waters, along the coast of Maine, nor in the Bay of 
Fundy. 
All the various forms of this species, upon which the several nominal 
species, united above, have been based by Lamarck and others, often 
oceur together in the same beds in Long Island Sound, and may easily 
be connected together by all sorts of intermediate forms. Even the 
same specimen will often have the form of borealis in one stage of its 
growth, and then will suddenly change to the Virginiana style, and, 
perhaps, later still, will return to the form of borealis. Or these differ- 
ent forms may be assumed in reverse order. Great variations in the 
number and size of the costz and undulations of the lower valve occur, 
both in different specimens from the same locality, and even in the 
same specimen, at different stages of growth. All these variations 
occur in precisely the same way in the shells taken from the ancient In- 
dian shell-heaps along our entire coast, from Florida to Maine. 
TUNICATA. 
SACCOBRANCHIA. 
CIONA TENELLA Verrill. (p. 419.) 
American Journal Science, ser. iii, vol. i, p. 99, ‘figs. 12, 13, 1871. Ascidia tenella 
Stimpson, Proc. Bost. Soc. Nat. Hist., iv, p. 228, 1853; Inv. of Grand Manan, p. 
20,1853; Binney, in Gould, Invert., ed. ii, p. 24,1870. ?Ascidia ocellata Ag., 
Proc, Amer. Assoe. for Adv. Sei., ii, p. 159, 1850 (description insufficient); Bin- 
ney, in Gould, Invert., ed. ii, p. 24, Plate 24, fig. 332, 1870. 
Cape Cod to Gulf of Saint Lawrence; rare and local south of Cape 
