346 AEACHNIDA. EURYPTERIDA 



149, vii. — xii.) ; the six posterior segments are without 

 appendages and form the metasoma (fig. 148, 7 — 12 ; fig. 

 149, xni. — xviii.), at the end of which is the tail-plate or 

 spine (g) ; this is sometimes {Eurypterus) spine-like, but 

 usually in the form of an oval plate which may be pro- 

 duced into a median spine as in Slimonia, or divided at 

 the end as in some species of Pterygotus. Each segment 

 of the prosoma is covered by a broad, slightly convex 

 dorsal shield (or tergum), and by a ventral cuticle (or 

 sternum), and the tergum of each segment overlaps the 

 one next behind. In the metasoma each segment is 

 surrounded by a continuous chitinous sheath. 



The mouth is on the under surface of the prosoma 

 (fig. 149). In front of the mouth there is one pair of 

 appendages only (i.), which end in chelae and are usually 

 small. The other five pairs of appendages (n. — VI.) are 

 at the sides of the elongate mouth ; they consist of from 

 six to eight joints each, and are not chelate ; they 

 functioned in locomotion, and also in mastication since the 

 inner margins of the basal joints (or coxae) are provided 

 with tooth-like processes ; the posterior pair (vi.), except 

 in Stylonurus, are much larger than the others and 

 have a very large basal joint. Placed just behind the 

 mouth, in the median line, is an oval or heart-shaped 

 plate, the metastoma (b), which covers the inner parts of 

 the basal joints of the sixth pair of appendages. The 

 metastoma represents the pair of chilaria of Limulus 

 (p. 340), and the presence in some cases of a notch in 

 front, and a median longitudinal groove on the surface, 

 supports the view that the metastoma originated from a 

 paired structure. Immediately in front of the mouth 

 another plate, the epistoma, is found in Pterygotus (fig. 

 148, a). 



