As Arca pectunculoides var. orbiculata Dall, 1881:
Examination of a large number of specimens in the Jeffreys collection has convinced me that the single valve described as variety orbiculata is merely an extreme variety of the typical pectunculoides, and not distinct, as I suspected then. It is, however, certain that all the American specimens are shorter and rounder than those from farther east in the Atlantic sea-bed and the Norwegian and arctic seas.
Arca grenophia Risso may be this species, but it was not figured, and the description is quite insufficient. Arca pectunculoides, var. crenulata Verrill, appears to have the form of var. orbiculata, the teeth of the Gulf specimens above mentioned, the marginal crenulations of glomerula, and the sculpture of the type of pectunculoides. I have only seen one right valve of crenulata, but both valves seem to be sculptured alike.
By a slip of the pen, in treating of Area glacialis Gray, Prof. Verrill (Trans. Conn. Acad., V. 576, 1882) represents me as recording A. glacialis from the Gulf of Mexico. This is an error ; as, in mentioning it in the Blake Preliminary Report (1. c, p. 121), I reported the pectunculoides (with which Prof. Verrill had seemed disposed to unite glacialis) from the Gulf, but expressly objected to its identification with glacialis, which I do not know from that region.
Habitat of the variety : Gulf of Mexico, Station 33, 1568 fms. (one valve).
Typical form : Sigsbee, off Havana, in 480 fms.; Station 16, near Havana, in 292 fms., living, bottom temperature, 56° ; Station 176, near Dominica, in 391 fms.; Station 211, near Martinique, in 357 fms.; and Station 230, off St. Vincent, living, in 464 fms., bottom temperature 41°.5 F.