376 THE OSTEACODERMS. 



ing to the olfactory chamber. When the outer end of the ethmoid was pushed 

 forward against the posterior wall of the rostral plate, two narrow passages -would 

 still remain open, leading from the olfactory pits to the exterior. (Fig. 259, A.) 



The olfactory pits of Bothriolepis correspond to the antorbital fossse of the 

 aspidocephali, indicating that the union of the median and lateral eyes and the 

 olfactory organs into a compact median dorsal group of sense organs is very 

 characteristic of the ostracoderms. 



Sensory Grooves. — The cutaneous sense organs were located in distinct open 

 grooves. (Figs. 259, A, 262.) There is a main suborbital, r.l., united in front 

 in adults, but separate in the young; a V-shaped orbital line usually connecting 

 with the suborbitals in front, and at the posterior end, in immature specimens, 

 leading into the endolymphatic ducts; a V-shaped posterior branchial, b.l., and a 

 lateral line, /./., extending backward along the sides of the branchiocephalon, 

 onto the sides of the trunk. 



pi. 



Fig. 25b. — Cross-section of Bothriolepis in the branchial region, showing the seven pairs of plate-like gills. 



The cephalic appendages are large, completely armored, and consist of two 

 movable joints. The proximal one is covered with six plates; it is triangular in 

 cross-section, rounded dorsally, flat ventrally, with a sharp tuberculate anterior 

 margin and a thick, flat, posterior one. (Figs. 247, 248.) An opening at the 

 proximal end of the posterior wall and an adjacent opening in the side of the 

 branchiocephalon served for the passage of blood-vessels and nerves into the 

 interior of the appendage. (Fig. 262.) The dorsal and ventral proximal plates 

 formed a rounded articular head that fitted into a socket in the anterior ventro- 

 laterals. From the center of the socket a fixed bony rod with an expanded 

 head projected into the interior of the appendage, thus holding it in place after 

 many of the softer tissues had completely macerated. (Fig. 254, a.) This bony 

 rod is continuous with an axial plate, or bar, of fibrous or cartilaginous tissue 

 that extends into the proximal end of the appendage. The distal joint of the 

 appendage was movable in a horizontal plane only. It-was hollow, oval in cross- 

 section, with recurved marginal spines, and covered with numerous polygonal 

 plates. 



