Structure and Classification of the Asterolepidc«. 487 



[l. o.) ; two antero-lateral, passing at an angle forwards and 

 inwards , articulating with the lateral plate on each side ; 

 and one anterior, retracted in the centre, so as to form a wide 

 reentering angle in which the correspondingly angulated pos- 

 terior aspect of the postmedian plate {id. m.) is received. On 

 each side of the median occipital is the lateral occipital {l. o.), 

 of an irregularly pentagonal shape and having one side in- 

 ternal, ardculating 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 [acj.). The 

 latter (postraarginal, 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 elongated plate, the post- 

 median [pt. m.), broadest in the middle, narrow at each end. 

 Its gently convex anterior margin forms the posterior boun- 

 dary of the median opening ; its posterior margin, obtusely 

 angulated, fills up the reentering angle of the front of the 

 median occipital, while each narrow extremity or outer margin 

 articulates with the lateral cranial plate. 



Each lateral plate (/.) is of an antero-posteriorly dongated 

 form and may be described as having four margins. The 

 very irregular inner one articulates with the median occipital 

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

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

 premedian [p.m.); the outer margin, slightly concave, arti- 

 culates with extralateral [e. l), the posterior with the 

 external occipital, while the short anterior one forms part ot 

 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 Pan- 

 der, "marginal" by Owen; but I prefer to call it extra- 

 lateral. Forming the lateral margin of the buckler external 

 to the lateral plate on each side, this element attains a 

 large size in Fterichtlii/s, and seems to have been only loosely 

 articulated 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. Inlig. 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. (Compaie Pander's figure of the same plate in 

 Asterolepis (7, tab. vi. fig. 1, no. 3).) 



I have seen no trace in Pterichthys of the narrow plate 

 which Pander figured in Asterolepis (7, tab. vi. fig. 1, no. 2) 



