THE ANTIARCHA. 375 



Hyoid Arches. — Back of the mandibles, the circumoral membrane is 

 strengthened by two bands of dermal armor. They are thin and delicately orna- 

 mented , and when the head is in a normal position the posterior band is entirely, and 

 the anterior one partly, overlapped by the anterior ventral plates. The narrow 

 anterior band consists of five or six segments. The posterior one is unsegmented. 

 Both bands are attached to a large lateral plate, the lateral end of which is bent 

 at right angles, and attached to the lateral walls of the head. (Figs. 253, 254, h.m.) 



The mouth was a slit-like opening situated between the premaxillae and in 

 front of the membrane uniting the two mandibles. 



The oral membrane with the attached premaxillae and mandibles could be 

 protruded a short distance, or withdrawn into the broad but shallow pre-oral 

 chamber. 



The Eyes and Olfactory Organs. — The oval opening on the anterior part of 

 the dorsal shield contains the stalked lateral eyes, the parietal eye, and the olfactory 

 organ. These organs were wholly or partly surrounded by small bony plates 

 held in place by tough but flexible membranes. (Fig. 255.) 



The lateral eyes were enclosed in short, rounded eye stalks that were attached 

 to the margin of the parietal and olfactory plates by hinge-like joints so that the 

 crescent-shaped eye openings on the distal ends of the stalks could be raised or 

 lowered into the orbits. The exoskeleton of each eye stalk consisted of a thick 

 posterior dorsal, d.s., a large anterior ventral, a.s., and two ventro-lateral plates, 

 l.s. When the posterior dorsal plate was level with the shield, the corneal opening 

 was concealed within the orbits. 



Parietal Eyes. — Between the lateral eyes is a thick quadrangular plate that 

 is nearly perforated by a deep conical pit, opening inward, and covered externally 

 by a thin-walled, lens-like tubercle. This pit contained the anterior parietal eye. 

 There are two similar pits, but not so deep, on the inner surface of the small post- 

 orbital. There is no indication of their presence on the external surface. 



Olfactory Organs. — The posterior end of the rostral plate divides into an 

 inner and an outer lamella, enclosing a wide triangular chamber between them. 

 (Fig. 251, r.) Just above the edge of the inner lamina is a small, T-shaped 

 ethmoid. It stands nearly vertically, with its dorsal transverse bar attached to 

 the anterior edge of the pineal plate. (Fig. 255, e.) When the anterior face of 

 the ethmoid is exposed, it is seen that its expanded arms, to which are attached 

 rod-like lateral ethmoids. I.e., and the pedicle form the lateral walls of two rounded 

 depressions in which the paired olfactory sacs were probably located. In the 

 bottom of each pit is a circular opening, leading into the interior of the head, 

 and serving for the passage of the olfactory nerves. 



All these plates are held together and attached to the sides of the sensory 

 opening by tough but flexible membranes, leaving a relatively large space for 

 the movements of the organs. It is clear, from the various positions of the plates 

 in different specimens, that the parietal plate could move caudally for several 

 millimeters, drawing the ethmoid backward and upward, thus enlarging the open- 



