Dr. R. R. Traquair — A New Genus of Coccosteidce. 57 



derived from the lines radiating from the ossific centres in a 

 specimen with the surface rubbed off, there can be no doubt that 

 the true form and arrangement of the constituent plates of the 

 shield has been arrived at. The pattern in two shields has been 

 represented in outline in PI. III. Figs. 1 and 2, and if the reader 

 will compare these figures with that which I have already given of 

 the cranial shield of Coccosteus, 1 the fundamental agreement as well 

 as the essential difference between the two will at once be perceived. 



The median occipital plate (m.o.) is more or less five-sided, 

 elongate, truncated behind, pointed in front, where it is wedged in 

 between the 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 occipital (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 which 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, 2 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. Fig. 2 represents the 

 configuration of a specimen which is proportionally 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 commences on 

 each side in the external occipital plate near the postero-lateral angle 

 of the shield. Running forwards and slightly outwards, it passes on 

 to the marginal plate, where it gives off a branch backwards and 

 outwards to the edge of the shield just behind the postero-external 

 angle. The main groove then turns forwards and slightly inwards 

 at an obtuse angle, and on passing on to the post-orbital turns on the 



1 Geol. Mag. Dec. III. Vol. VI. PI. I. Fig. 2. 



2 Op. cit. pi. ix. tig. 1. 



