the second order, the 
first of which are 
included, the outer 
end meeting the low- 
er primary segment 
wall, while the young- 
er segments of the 
second order reach 
the border of the 
Ce One frond. The number 
SOS Ve Se f included segment 
SO OSD ADO at Die ol included segments 
ID OP CP ESR EOI ESS C of the second order is 
FES variable, it is great- 
Fig. 445 est in the broader 
Membranoptera alata. Tip of frond from Frederikshavn, May. forms (comp. Kny 
1. c., K. ROSENVINGE 
1913 p. 22 (from Cherbourg) and Kyrın 1923 fig. 70), while in the narrow forms 
from the Baltic Sea it may be only 1 or 2 (fig. 447). Their number can be recog- 
nised in the older parts of the frond by the 
aid of the lateral veins of the second order 
arising in the upper border of the segment 
Fig. 447. 
Membranoptera alata f. baltica, 
from Bornholm. Tip of frond. 
350 : 1. 
ofthe second order 
(fig. 446 A), just as 
the lateral veins of 
first order arise in 
the upper border 
of the primary 
segments. In the 
branch - producing 
segments one seg- 
ment of the second 
order only is asa 
rule included (fig. Fig. 446. 
445.1 br I Membranoptera alata. A, tip of frond showing the 
Dp HOES ONE mid-rib and the lateral veins. B, portion of membrane 
on the right hand; showing the veins. 19:1. 
comp. Kny l.c., se- 
cond branch from the top on the right; K. R. 1913 fig. 158 
last branch; Kyrın 1923 fig. 70 a). But it also happens that 
the first segment of the second order reaches the border of 
the frond (fig. 445 last branch on the right, comp. Kny L.c. 
last branch on the right, Kyrın 1923 fig. 70 c). When the 
first segment of the second order of the branch-producing 
