116 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY 
42. 
Libellula semifasciata 
51. 
Sympelrum obtruaum 
43. 
riatheinis lydia, one record ^- 
52. 
rubicundulum 
Chathain 
53. 
vicinum 
44. 
Perithemis dornitia tenera 
54. 
Leucorrhinia frigida, one record 
45. 
Nannothemis bella 
— Hyannis 
46. 
Erythrodiplax berenice 
55. 
intacta 
47. 
I'jrythcmi.s simplicicollis 
56. 
Celithemis elisa 
48. 
Pachydii)lax longipcnnis 
57. 
monomelaena 
49. 
Sym))etriiin corruptum 
58. 
ornata 
50. 
coHtiJerimi 
59. 
Tramea Carolina 
On the Cape itself another interesting control is to be noted. A 
few species that inhabit the deeper, ice-block kettles are absent 
from the shallower out wash ponds, and vice versa. Here again the 
mud of the former as compared with the sand of the latter prob- 
ably explains the larval distribution of the different species. 
One other factor of interest, already noted, is that the most boreal 
forms, if appearing in unusually southern stations, are always of 
early spring or autumn occurrence. Williamsonia lintneri, Enal- 
lagma calverti, and others at the southern limit of their ranges 
appear in April, May, or early June. This would suggest the im- 
portance of temperature as the most potent distributional control. 
There is in New England at least, however, one interesting in- 
stance of distribution that seems most probably explained by geo- 
logic, rather than by topographic or climatic influence. Prof. M. 
L. Fernald has pointed out that ''the Tertiary continental shelf 
... is now depressed as a shallow bench off the east Atlantic 
coast of America; and from the botanical and zoological evidence, 
as well as from recently published geological evidence, it now 
seems perfectly settled that the continental shelf formed in the 
late Pleistocene and even later a nearly continuous although some- 
what interrupted floor from New Jersey and southern New Eng- 
land, by way of Sable Island and the Grand Banks, to southern 
and eastern Newfoundland" (Amer. Journ. Bot., 5 : 238, 1918). 
Without citing the botanical and zoological evidence to which he 
refers, and which seems steadily accumulating, the Odonata supply 
another seemingly definite instance. The two holarctic species, 
Enallagma calverti and E. cyathigerum, bridging a wide hiatus in 
southern New England from the boreal regions, occur again on 
the coast from Province town to Point Judith, R. I., and on Nan- 
tucket. Both species have been recorded from Newfoundland as 
