122 WILLIAM PATTEN. 



A broad thin shelf of bone extends inwards from the ant- 

 orbital plate forming a wide chamber between the anterior surface 

 of the T-shaped bone described by Whiteaves, or the ethmoid as 

 I shall call it, and the two arms of the ant-orbital, Fig. 2. When 

 the anterior face of the ethmoid is exposed, it is seen that its 

 arms nearly enclose two circular openings, <?/, in which the floor 

 of the olfactory organ was probably situated, and which also 

 served for the passage of the olfactory nerves. 



These two pits clearly correspond to the antorbital fossae of 

 Ccp/ialaspis and Tremataspis, and thus a more satisfactory ex- 

 planation of the location of the olfactory organs is furnished 

 than the suggestion that they are situated on the margins of the 

 mandibular plates, where there are no indications whatever of 

 any infoldings of the oral membrane that might be regarded as 

 olfactory organs. 



Two large saucer-shaped sclerotic plates are attached by short 

 movable arms to the lateral arms of the ethmoid, a. s., and two 

 much thinner and crescent-shaped plates, /. s. and p. s., form the 

 lateral and posterior walls of the orbits. The well known plate 

 attached to the sides of the pineal plate, ds, completes the dorsal 

 wall of the orbit. 



All these plates are held together, and to the sides of the sen- 

 sory opening, by tough but flexible membranes, leaving thus 

 a relatively large space for the movements of the various parts. 

 It is clear from the position of the plates in different specimens 

 that the pineal plate could be pushed backward for several milli- 

 meters drawing the ethmoid backwards and upwards so as to 

 open the olfactory chamber. This movement would also raise 

 the anterior and lateral sclerotics which in turn would elevate the 

 dorsal sclerotics and expose the eye openings, o, on the sides 

 and front. The reverse movement would lower the ethmoid and 

 draw the corneal surface of the eye into a pocket formed by the 

 thick median margins of the adjacent cranial plates. Thus the 

 eyes and olfactory pits could be opened and closed by the same 

 set of movements. 



The sensory canals that converge toward the median occipitals 

 do not unite there as usually figured. Their posterior ends sep- 

 arate close to the median line and terminate in two small open- 



