THE OSTRACODERMS. 



aspid, probably without orbits, and in which the exoskeleton tends to break 

 up into separate polygonal plates, which correspond to the polygonal areas on 

 the continuous shield of Limulus and Cephalaspis. (Fig. 199.) 



II. THE ANASPIDA. 



The anaspida include a small number of obscure forms. They were com- 

 pletely covered with small dentinal plates, without multipolar lacunae, which prob- 

 ably represented the fragmented outer layer of the primitive exoskeleton. Orbits 



FIG. 243. A, Restored outlines of Lasanius problematicus, Iraq ; B, Birkenia elegans, Traq; C, Thelodus 

 scoticus Traq.; D, Lanarkia spinosa Traq.; E, sagittal section of a primitive dermal denticle (Coelolepis Schmidti) 

 A-D. after Traquair; E after Rohon. 



may be covered with bony plates. Small marginal openings indicate the 

 location of gill clefts or the points of attachment of the cephalic appendages. 



Coslolepidae. Resembling somewhat the cephalaspids in form, and covered 

 with separate rounded, or conical, denticles. Upper Silurian, passage beds. 

 Thelodus. (Fig. 243, C.) Lanarkia. (Fig. 243, D.) 



Birkeniidae. Fishlike contour, with oblong tuberculate plates. A series of 

 branchial openings, like those in Tremataspis. Birkenia. (Fig. 243, B.) La- 

 sanius. (Fig. 243, A.) 



III. PTERASPIDA. 



Head sagittate, or oval, consisting of a small number of large plates. 

 Subdivisions of head united, forming a common cephalic buckler without 



