1891. 



DERMAL PLATES OF HOMOSTEUS. 



199 



gradually diminish to the anterior third of the bone and then pass 

 into broad undulations bounding a median longitudinal hollow. 

 Posteriorly, where the two lateral occipitals approach most closely, 

 the ridge is an arrowhead-shaped boss, the apex directed back- 

 wards and the base forwards ; and at this point there may have 

 been a descending plate of bone. At its bifurcation the ridge 



\ 4 



Figs. 1-4. Dermal plates of Homosteus milleri, from the Caithness Flagstones; 

 all shown from the visceral aspect, of one quarter the natural size. 

 1, median occipital ; 2, left lateral occipital ; 3, anterior median 

 ventral; 4, impression of left anterior ventro-lateral. /, facettes 

 for overlap. 



becomes much less elevated, broader and ronnder, gradually dimin- 

 ishing forwards as just described; and on either side is a series of 

 shallow, transverse linear grooves, symmetrically disposed in several 



