CEPHALASPID^. 



:37 



between and in front of tlicm, are very constant in the Ostcostracous shields. The 

 structnres alhulcd to arc seen in the woodcut (fig. 11), which has been carefully constructed 

 from a remarkably well-preserved concave cast of the parts in (piestion of a specimen of 

 Ceplialaspis Agassizii. Between the orbits is a well-marked tubercle, which I call the 

 interorhital jnominence («./;.). In front of each orbit the material of the test is also 

 raised into a projecting mass, which is to be called Ulc atif orbital prominence (a.^j.). 

 Between the two antorhital prominences is a very deep depression of the shield, 

 divided beneath the superficial lamina into two parts by a narrow septum continued 

 from the interorbital prominence^ which is hollow. The two cavities thus formed 

 (see also Plates IX, fig. 2, and PI. XIII, fig. 4 a) are the antorhital fossce {a.f.)} 'i'he 

 material of the shield forms a somewhat elevated ring round each eye, which may be 



Fig. 11. Fig. 12. 



»». C. r a.f. 



V. a. p. r. p. .1. p. a. 



Restored outline sketch of the head-shield of Diagiamniatic view, from below, of the cephalic shiehl 



Eucephalaspis ^yassizii. of a Eucephalaspis. 



./.antorhital fossa-, i. y;. interorbital prominence, o. r. orbital rings, i. p. interorbital groove, o. ». post -orbital valley, m. c. marginal nil 

 p. c. posterior cornii,/>. a. posterior angle, /<.r. posterior ridge,/;. posterior spine, r. rira or margin. 



designated the orbital ring (o.'r.). Posterior to the interorbital prominence, and more strictly 

 placed between the two orbits, which in some specimens it has the appearance of uniting, 

 like the nose-saddle of a pair of spectacles, is a deep well-defined impression, which 

 may be distinguished as the interorbital groove {i. g.). Immediately behind this, and 

 reaching backwards towards the posterior spine, as it approaches which it narrows, 

 ' These occupy such a position as to suggest a connection with the olfactory organ, whici) hence may 

 be inferred to have been double in these fishes— a fact which makes it certain that, tliough these are the 

 earliest fishes yet found, they are very far indeed from the frst Vertebrate*, their ancestors or forerunners ; 

 they are also, it seems likely, far in advance of the ' Protamphirine ' of Ilaeckel. 



