170 SUPPLEMENT TO THE CRAG MOLLUSCA. 
geographical extension. It is found in the seas of Britain, in the Mediterranean, on the 
shores of North America, also on the coast of Japan. It (or at least a shell not to be: 
distinguished from it) occurs likewise in the southern hemisphere, and in the Pacific ; 
indeed, it is almost difficult to say where it is not to be found. It has also a bathy- 
metrical range from low-water mark into water of abysmal depth. 
This ubiquitous animal has a special claim to our attention, as it seems to offer a 
defiance to our determinations. Conchologists are perplexed in their examinations, and 
fail to separate specimens from opposite hemispheres, north and south, east and west, by 
any distinctive marks usually denominated specific. The late Edward ¥orbes was inclined 
to believe that the shell called 7. strata (‘ Geol. Rel. of the Existmg Fauna and Flora of 
the Brit. Isles,” p. 73), found in the Chalk and Green Sand presented differences so 
insignificant from those of the existing caput-serpentis that he doubted whether the one 
was specifically distinct from the other. Mr. Davidson, after having thoroughly examined 
the fossils called striata and striatula with caput-serpentis, inclmes to the opinion that 
they are all three distinct ; but he acknowledges that the differences are but trifling, and 
even hesitates to give a decided opinion. I have examined carefully these species, and 
seen the differences pointed out; but unless they are allowed to have more importance 
than differences of equally slight degree are allowed to have in other families of Mollusca, 
I much doubt if the three forms be entitled to specific separation. The fossil species from 
the Upper Secondaries and Lower Tertiaries have generally been considered by palzon- 
tologists as specifically distinct ; but it may be asked whether the great difference in age 
of the several deposits in which they have occurred may not in some degree have influenced. 
paleontologists in their determinations. If the Green Sand shell be really identical with 
T. caput-serpentis it has the greatest antiquity of any Mollusc at present known, and, what 
‘s also remarkable, the existing shell exhibits no symptom of specific decline, as it flourishes 
in our own seas at the present day in great profusion; and its world-wide diffusion and 
great bathymetrical range seems almost confirmatory of its ancient origin. 
ARGIOPE CISTELLULA, S. Wood. Supplement, Tab. XI, fig. 4 a—d. 
TEREBRATULA CISTELLULA, S. Wood. Catal. Ann. and Mag. Nat. Hist., vol. v, p. 253, 
1840. 
ARGIOPE — Davidson. Brit. Tert. Brach., p. 10, 1852. 
Diam. =*5 mech. 
Locality. Coralline Crag, Sutton. Recent, Britain. 
This does not appear to be abundant anywhere either as a fossil or as a recent species. 
I have found about twenty specimens in the Coralline Crag, and these show a considerable 
variation in outline, especially in the elevation of the umbonal portion of the larger 
valve. 
