748 Rev. C. R. N. Burrows on the 
excavation. The@enitalia of the females of H. paludis 
and H. lucens are extremely difficult to separate. There 
appears to be no absolutely certain difference except that 
the genital plate is perhaps more substantial in HZ. lucens 
than in H. paludis, and has also in H. lucens a tendency 
to fold in the centre, doubling over and presenting the 
appearance of a dark central line, which effect I ascribe 
to the plate being more full than in H. paludis. 
In H. crinanensis the genital plate is wide and deep. 
The central excavation is extremely narrow and very deep, 
extending more than half-way through the plate. The 
lateral creases are above the widest part of the plate. 
The lodix has only the smallest possible central triangular 
excavation. 
In #7. americana the genital plate is wide and deep, the 
central excavation being V-shaped. The lateral creases, 
diverging from about the centre of the side of the plate, 
extend obliquely in a straight line until they are lost in the 
posterior margin of the ninth segment. The excavation 
in the centre of the lodix appears as a wide sweeping curve. 
(The specimen from which this description is taken is 
reasonably assumed to be H. atlantica.) 
The number of specimens examined for these details of 
genitalia formation is as follows :— 
Number of specimens Mounted. Examined 
examined. Male. Female. 
TET ICTULOAUS pane ae eae 25 8 60 
ENA TNAIS Ss) Oe foe ihe 25 8 30 
VERA Ucenset, secon. Une 2, 1 100 
EICTrUNONETISISIN tae 8 5 96 
H. atlantica . Tey: 7 1(2) 
HEL CSTOLICOs seam 1 
These differences in the appendages cannot be referred 
to geographical variation as, apart from their being too 
considerable, our four British species, though each having 
a special area of distribution (except nictitans, which is 
very widespread), have unquestionably no impediments 
to freely crossing with each other. But there are no 
indications that such ever takes place, there being no 
specimen at all intermediate discovered amongst all the 
examples examined. 
In Acronicta psi and tridens we have a very similar case, 
affecting two species; the group before us presents eight. 
Psi and tridens have very different larvae. In the species 
before us we have had practically nothing done in the way 
