Homosteus, Asmuss, compared with Coccosteus, Agassiz. 53 



perfect manner, and from this model the drawing has been 

 taken. And I may add that every detail of the buckler here 

 given is corroborated by another splendid specimen, also 

 from the collection of John Miller, in which, however, the 

 dorsal plate has got displaced to one side. 



We find a wonderful correspondence in the arrangement 

 of the bony plates, the differences appearing almost entirely 

 due to the altered position of the eyes, and the assumption 

 by the cranial shield of an antero-posteriorly elongated 

 instead of a broadly hexagonal figure. 



The median occipital (in. o.), preserving its trapezoidal 

 shape, has become much elongated, as have also the external 

 occipitals (e. o.), while the centrals (c.) have become much 

 smaller in proportion, and have come to take part in the 

 inner boundary of the orbit, more, however, on the upper 

 than on the under aspect ; they are also in contact in front 

 with the p)Osterior ethmoidal (j:?^. e), the hinder angle of 

 which is inserted in a notch between their anterior ex- 

 tremities. The marginal (m.), also much elongated, is easily 

 recognised ; but it is now alongside of, instead of in front of, 

 the external occipital, and the p)<)^l-orhital {pt. o.) and 27re- 

 orhital {p. o.) have altered their relation to the orbit in 

 a strange fashion. Separating internally, so as to allow the 

 central to come into the boundary of that opening, they have 

 thrown out processes which unite externally, and so in 

 Homosteits the orbits come to be entirely enclosed within the 

 buckler, instead of being outside it, as in Coccosteus. The 

 last plate to be noticed is the anterior ethmoidal (ci. e.), which 

 occupies a position at the front of the snout exactly as in 

 Coccosteus, but no trace is seen either of the small pre- 

 maxillary, or of the nasal openings of that genus. 



The lateral-line system of grooves is sparingly developed 

 on the cranial shield of Hoinosteus. On each side the lateral 

 groove passes along the external occipital and the marginal 

 on to the post-orbital, where it divides into two branches, 

 one of which passes outwards and forwards to the margin of 

 the shield, while the other passes backwards and inwards to 

 be lost near the middle of the central plate. If we compare 

 this arrangement with that in Coccosteus (Fig. 2), we shall see 



