288 ARACHNIDA——EURYPTERIDA CHAP, 
first six form the mesosoma (Fig. 161, 7-12). The tergum on 
the dorsal surface of each segment is broad and short, the middle 
part being slightly convex and the lateral parts slightly concave ; 
the external margin is bent under, thus forming a narrow rim 
on the ventral surface. The tergum of each segment overlaps 
the one next behind. The segments increase in breadth shghtly 
up to the fourth segment, posterior to which they gradualiy 
become narrower. 
On the ventral surface the segments of the mesosoma bear 
pairs of plate-like appendages, each of which overlaps the one 
behind like the tiles on a roof. On the posterior (or inner) 
surfaces of these appendages are found the lamellar branchiae, 
which are oval in outline (Fig. 165, d). Between the two 
appendages of the first pair is a median process which is genital 
in function; this pair are larger than the other appendages, and 
cover both first and second segments, the latter being without 
any appendages, and they represent the genital operculum of 
Limulus (Fig. 153, 10). The form of the operculum, more par- 
ticularly of the median process, differs in the male and female. 
In that which is believed to be the female (Fig. 162) the median 
process is long, and extends beyond the posterior margin of the 
operculum ; it is formed of two small five-sided parts at the base 
which are united at the sides to the two plates of the operculum ; 
behind this is a long, unpaired part, which is pointed in front ; 
this, together with the remaining parts, is not joined to the 
side-plates of the operculum, so that the latter are here separated 
from one another. The third part of the median process is 
shorter than the second, and bears at its end a pair of small 
pointed and diverging plates, the tips of which reach to the 
middle of the third plate-like appendages. On the inner side of 
the operculum there are, in the female, a pair of curved, tubular 
organs, attached to the anterior end of the median process, where 
they open, the free ends being closed; the function of these 
organs is not known, but was probably sexual. 
In the male (Fig. 163, A, a) the median process is formed of 
two parts only, and is very short, so that the two plates of the 
operculum unite behind the process. 
In the female a median process (Fig. 163, B) 1s also present 
between the second pair of appendages (belonging to the third 
segment of the mesosoma); it consists of a basal unpaired part, 
