698 EEPOET OF COMMISSIONER OF FISH AND FISHERIES. 



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 dag up from deep beneath the mud in tlie harbor of 

 Portland, Maine, in large quantities, but native oysters appear to be 

 entirely extinct in Oasco 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 ■ Eiver, and in Tampa Bay, Florida (Oonrad). Texas 

 (Roemer). 



Fossil in the Post-Pliocene at Point Shirley, Massachusetts, 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 interrn/pta, 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 

 occur 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 horealis in one stage of its 

 growth, and then will suddenly change to the Virglniana 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 costse 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. 



TUN OATA. 



SACCOBRANCHIA. 



Cigna tenella. Verrill. (p. 419.) 



American Journal Science, ser. iii, vol. i, p. 99, figs. 12, 13, 1871. Ascidia tvnella 

 Stimpson, Proc. Bost. Soc. Nat. Hist., iv, p. 228, 1853 ; Inv. of Grand JSIanan, p. 

 '/^O, 1853; Binney, in Gould, Invert., ed. ii, p. 24,1870. fAscidia oceUata Ag., 

 Proc. Anier. Assoc, for Adv. Sci., ii, p. 159, 1850 (description insutticient) ; Bin- 

 ney, in Gould, Invert., ed. ii, p. 24, Plate 24, fi«;. 332, 1870. 



Cape Cod to Gulf of Saint Lawrence; rare and local south of Cape 



