HETEROSTRACI. 



d 



FIG. 4. 



Ptetaspis rostrata ; diagram of dorsal shield, external aspect, about one-third 

 nat. size. L. Old Bed Sandstone ; Herefordshire district, c, cornua, with 

 branchial cleft across hinder portion ; d, median disc ; o, orbit, surrounded 

 by small orbital plate ; r, rostral plate ; s, posterior dorsal spine fixed in 

 cleft of median disc. (After Lankester.) 



FIG. 5. 



Pteraspis crouchi ; imperfect dorsal shield (median disc), with most of the 

 external layer destroyed, exhibiting the sensory canals infiltrated by dark 

 mineral matter, nat. size. L. Old Red Sandstone; Herefordshire. 



FIG. 6. 



Pteraspis rostrata ; restoration, lateral aspect, about one-third nat. size. 

 L. Old Red Sandstone ; Herefordshire district. (From Brit. Mas. Catal.) 



