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 slightly 

 up to the fourth segment, posterior to which they gradually 

 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, cT). 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) is also present 

 between the second pair of appendages (belonging to the third 

 segment of the mesosoma) ; it consists of a basal unpaired part, 



