240 BULLETIN OF THE 
tracery, usually white inside, but occasionally very dark brown, this character 
being apparently local; the concentric wrinkles are close but less strong, a 
little wear makes them seem absent, and the shell smooth ribbed ; the same 
differences exist as to truncation ; this character is probably sexual. 
The variety carinatus has the same number of ribs as the typical form, but 
they are carinated, and the interspaces toward the margin, owing to impressed 
radiating lines, seem to have several small threads in them between the ribs ; 
the concentric wrinkles are more distant, and take a lamellate aspect, forming, 
with the ribs, a reticulation which seems very characteristic ; the shell is a 
little more globose than the ordinary form, but not much ; otherwise it seems 
precisely the same, and all the gradations, from flat wrinkled ribs to keeled 
and reticulated ones, may be seen in the series before me. A single one taken 
by itself would certainly appear distinct from the ordinary form, and this gives 
us a hint of what we may expect when large numbers of specimens come to be 
studied scientifically and with due regard to their geographical distribution. 
Genus ARCA Linne. 
Arca pectunculoides Scaccui, var. orbiculata. 
Arca pectunculoides Seacchi, var. orbiculata, Dall, Bull. M. C. Z., IX. p. 121. 
Plate VIII. Fig. 5. 
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. 
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. erenulata 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 aslip of the pen, in treating of Arca 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 
