26 Proceedings of the Royal Physical Society. 



On each side of the median occipital is the lateral occipital 

 {I. o), of an irregularly pentagonal shape and having one 

 side internal, articulating with the median occipital; two 

 posterior, articulating with the anterior dorso-lateral (a. d. I.) 

 of the carapace ; one anterior, articulating with the lateral 

 plate of the head; and one external, joined to the angular 

 (ag.). The latter (postmarginal, Owen) is a very small plate, 

 also forming part of the cranial shield behind the lateral 

 plate. In front of the median occipital, which it entirely 

 separates from the median opening, is a transversely elon- 

 gated plate, the postmedian (pt. m.), broadest in the middle, 

 narrow at each end. Its gently convex anterior margin forms 

 the posterior boundary of the median opening ; its posterior 

 margin, obtusely angulated, fills up the re-entering angle of 

 the front of the median occipital, while each narrow extremity 

 or outer margin articulates with the lateral cranial plate. 



Each lateral plate (I.) is of an antero-posteriorly elongated 

 form, and may be described as having four margins. The 

 very irregular inner one articulates with the median occipital 

 and postmedian, is then notched to form the outer margin of 

 the median opening, in front of which it articulates with the 

 premedian {p. ra.) ; the outer margin, slightly concave, arti- 

 culates with the extralateral (e. I.), the posterior with the 

 external occipital, while the short anterior one forms part of 

 the front margin of the cranial shield. 



The remaining plate (e. I.) of the cranial shield is that 

 which in Asterolepis has been named " opercular " by Pander, 

 "marginal" by Owen; but I prefer to call it extralateral. 

 Forming the lateral margin of the buckler external to the 

 lateral plate on each side, this element attains a large size 

 in Pterichthys, and seems to have been only loosely arti- 

 culated to the side of the head, as it so frequently occurs 

 dislocated and removed from its position, while the other 

 cranial plates still cohere together. In Eig. 4 I have repre- 

 sented it in an isolated condition, where it will be seen that 

 its inner margin, which must have been largely overlapped 

 by the lateral plate, shows a peculiar notch a little in front 

 of the middle. (Compare Pander's figure of the same plate 

 .in Asterolepis (7, tab. vi., fig. 1, No. 3).) 



