230 Proceedings of the Royal Pkysical Society. 



wedged in between tlie hinder thirds of the two centrals. 

 Its ossific centre is near the posterior margin, and is marked 

 by a prominent elevation of the surface. On each side of 

 the median occipital is placed the external occijntal (e.o.), 

 which, although differently shaped from that in Coccosteus, 

 forms, as in that genus, the postero-external angle (P.E.) of 

 the shield. In front of these three occipital plates, and 

 occupying a position in the middle of the shield rather 

 nearer the front than the back, are the two central plates 

 (c), whose difference of form from those of Coccosteus is 

 equally striking, as is the case of the median occipital. 

 They are more or less of an ovate-oblong, approximating to 

 an elongated hexagonal form, articulating in the middle line 

 with each other and round about with all the other plates of 

 the shield except the rostral or anterior ethmoidal. The 

 marginal plate {m.) is situated on the outer side of the 

 central in front of the lateral occipital, and forms the 

 antero-external angle of the shield (A.E.) ; in front of it is 

 the post-orbital (pt.o.), which forms the post-orbital angle 

 (P.O.), and the posterior part of the orbital margin. The 

 front of the shield is now filled in by the pre-orbital plates 

 (P.O.) which meet in the middle line, form the ante-orbital 

 angle, part of the orbital margin on each side, as well as 

 the anterior median shallow excavation, in wliich the plate 

 named ''rostral" by Whiteaves fits. This rostral plate is 

 not present in any of the specimens in the Edinburgh 

 Museum, but its form and position in the specimen figured 

 by Whiteaves,* render it evident that it corresponds with 

 the anterior ethmoid in Coccosteus. 



Some amount of variation is observable in the form of 

 these shields as well as of their component plates. Eig. 2 

 represents the configuration of a specimen which is propor- 

 tionally shorter and broader than usual, and in which also 

 the median occipital plate advances further forwards between 

 the centrals, which are more irregular in shape, and have 

 their long axes divergent backwards. 



The arrangement of the lateral line system corresponds in 

 the main with that in Coccosteus. The lateral groove com- 



^ Oj). ciL, ])1. ix., iig. 1. 



