192 
ACTINOPTERYGII. 
to some extent the superior border of the orbital cavity; this in 
our nomenclature is evidently the squamosal (sq.). In the majority 
of Pycnodonts, the space between the supraoccipital, frontal, and 
squamosal elements is occupied on each side by a single parietal 
Fig. 30. 
Mesturus lecdsi ; diagram of bones ol cranial roof, superior aspect, from a 
specimen* in the collection of Alfred N. Leeds, Esq., Eyebury.—Oxford 
Clay; Peterborough. 
eth., ethmoidal plates; fr., frontal; m.eth., median ethmoidal plate; orb., 
orbit; sq., squamosal; s.occ., supraoccipital; s.t., supratemporal; opiates 
in parietal region. 
plate ; while this plate in Mesodcm, Microdon, Stemmatodns, Coelodus , 
Palceobalistum , and Pycnodus exhibits a large posteriorly-directed 
process with digitate extremity, arising about the middle of its hinder 
margin and extending immediately beneath the scales of the trunk. 
In Mesturus leedsi , however, the parietal region is occupied by 
two or more irregular plates on each side (a?); and neither in this 
genus nor in Gyrodus is there any posteriorly-directed process. The 
ethmoidal plates, as shown in Mesturus leedsi , are most remark¬ 
able (eth.). The region in advance of the frontals is covered with 
polygonal tesserae, which extend even over the smooth premaxillae: 
