CHIROCEPHALUS. 45 



and on the front part of the head, we see a small, black, 

 smooth spot, of a somewhat triangular shape, and which 

 is generally described as a third sessile, simple eye. It 

 appears to be merely the mark remaining of what was 

 the eye in the young state, and will be more particularly 

 mentioned when describing the young animal and its 

 transformations. 



The mouth is situated in the inferior surface of the 

 posterior cephalic segment, and consists of a lip, one pair 

 of mandibles, and two pairs of jaws. 



The lip (t. IV, f. G) is large, projecting, curved back- 

 wards, and is prolonged below the other parts of the mouth. 

 It is articulated, and has considerable motion, enlarging 

 and contracting alternately. It is the " soupape" of 

 Prevost and Jurine. 



The mandibles (t. IV, f. H) are also large, embracing 

 about four fifths of the circumference of the segment of 

 the head, to which they are attached, somewhat curved, 

 having the inner extremity large, obtuse, black, and 

 furnished at its edge with numerous very small teeth, so 

 fine as scarcely to be distinguished with the microscope, 

 while the other extremity is smaller, and terminates in a 

 sharp point. 



at greater length. " It consists," he says, " of four successive layers of 

 different kinds. The external layer is a smooth, homogeneous, transparent 

 cornea. Beneath it lies a facetted membrane, which, seated in a clear sub- 

 stance, contains rather darker, firmer, circular apertures, of equal size, and 

 regularly distributed in such a manner that every ring is surrounded by six 

 others, at equal distances from each other. The third layer of the eye con- 

 sists of egg-shaped, transparent, very hard lenses, each of which is situated 

 behind one of the little window-like apertures described, resting upon the 

 surface of the latter with its flatter extremity, and raising this a little with 

 that convex surface. The fourth layer consists of an oblong, club-shaped, 

 crystalline body, which encircles with its upper thicker end the more pointed 

 end of the egg-shaped lens, and is surrounded by a delicate membrane. A 

 continuation of this membrane also overspreads the lens, and attaches itself 

 to the thickened margin of the little aperture before each lens. Behind the 

 crystalline body there then follows the dark pigment as the principal mass 

 of the whole eye, through which the fibres of the optic nerves extend them- 

 selves to the respective ocelli, resting on the basis of the crystalline body, 

 and the lenses, and through these sheaths likewise attach themselves to the 

 facetted second membrane." Organization of Trilobites (Ray Soc. edit.), 

 p. 19. 



