342 



Mr. C. T. Regan on the 



between Coccostens and Pterichthys, then the frontals have 

 been displaced forwards and have either disappeared or 

 become fused with the ethmoid or with the suborbital plates, 

 and the so-called postmedian represents the parietals. On 

 the other hand, there is the possibility that this element may 

 be frontal in origin and that the median dermo-occipital may 

 include the parietals, and I incline to this latter view. 



Fi^-. 4. — Ventral plates of trunk-armour of (A) Pterichth/s (after Tra- 

 quair) and (B) CoccQsteus (after Traquair). i.L, interlateral ; 

 s., lateral spine; ji., pectoral limb; a.m.v., anterior median 

 ventral ; p.7n.v., posterior median ventral ; ci.o.I., anterior 

 ventro-lateral ; p.v.L, posterior ventro-lateral. The faint lines 

 indicate the extent of the overlap ; the suture between the 

 interlateral and the lateral spine in Coccosteus has been inserted. 



The arrangement of the plates of the armour of the trunk 

 is on a very similar plan in both Coccosteidte and Astero- 

 lepida?, 1 or 2 median dorsal plates, 1 or 2 pairs of anterior 

 and posterior lateral plates, and on the ventral surface 4 large 

 plates in exactly the same position and overlapping each 

 other and a smaller four-sided median piece in a very similar 

 manner, whilst a small anterior median plate may or may 

 not be present. The semilunars of the Asterolepidai seem to 

 correspond to the elements (interlaterals) which' have been 

 regarded as clavicles in the Coccosteidse. 



The case of Acanthaspis may be cited as evidence of the 

 similarity of the plates of these two families in structure and 

 arrangement. This genus, according to Smith Woodward *, 



* Vert. Palfeont. p. 16. 



