Fic, 12.—Diagram of Fig. 11. A, cephalic shield; Fic. 13.—Theoretical diagram of the upper side. 
B, thypostoma; © genal spines; D, tho- 
tanio aon buckle apse eA, 
Fic. 11.—Protolimulus eriensis (Williams). A sandstone cast ongitudinal ridges of the buckler; KK, 
of the under surface, Gata ne portions of the gnathopodes; L, ?folia- 
: ceous terminations of the last gnatho- 
podes; M, position of the mouth. 
PROTOLIMULUS ERIENSIS. After Williams. 
S. Mis. 154——To face page 150. 
