356 THE OSTRACODERMS. 



medullary tube, and the cerebral eyes of vertebrates that have been carried into 

 it during the early stages of development. Those ostracoderms in which the 

 lateral eyes appear to be absent are to be regarded as the ones whose newly- 

 formed cerebral eyes (phylogenetically speaking) have not become functionally 

 adjusted to their new environment. The concealed lateral eyes of larval cyclo- 

 stomes are in a similar condition, and they are to be explained in a similar manner. 

 See Chapter IX. 



The parietal eye was relatively large, and was lodged in a deep pit on the 

 under side of a projecting tubercle of the cephalic buckler (pteraspidians) , or on 

 the under side of a small movable plate, lying between the lateral eyes, (Tremataspis, 

 Cephalaspis, and Bothriolepis). In Bothriolepis there are two additional pits on 

 the inner surface of the postorbital plate, that probably contained another pair of 

 parietal ocelli. (Figs. 252, 253, 255.) 



The olfactory organs were probably located in a hypophysis-like median sac, 

 situated just in front of the orbits. In Tremataspis, Cephalaspis, and Thyestes, 

 the oval opening to the sac lies at the bottom of a shallow depression, that may be 

 a little deeper on either side. In Bothriolepis there is a small movable, T-shaped 

 plate (Fig. 255, e), that stands nearly vertically in the large opening common to the 

 median and lateral eyes. To the outer ends of the plate are attached two con- 

 cave, lateral wings, I.e., that appear to have partly enclosed the olfactory organs. 

 A narrow canal leads outward from each chamber, opening to the exterior just 

 in front of the top of the plate. 



Auditory Organs. — In Tremataspis and Bothriolepis there are two small, 

 sharply defined openings, situated close together in the occipital region, which 

 probably represent the outer ends of endolymphatic ducts. 



In Bothriolepis, when seen either in sections or dissections, they lose their 

 sharply defined walls just below the outer surface, and open into irregular cham- 

 bers that may lead either into the cancellous tissue or into the interior. There are 

 no definite openings corresponding to them on the inner surface of the shell, but 

 in etched heads there may be present a conspicuous spur representing the cast 

 of the inner opening. (Fig. 252.) In Tremataspis, the canals lead into small bony 

 tubes that project some distance from the inner surface of the shield. In Cyathas- 

 pis there are two V-shaped ridges in this region, that have been regarded as the 

 surface indications of semi-circular canals. 



The ducts are in some way related to the lateral line organs, for in young 

 specimens of Bothriolepis they mark the median termination of the orbital lines. 



Cutaneous Sense Organs. — A system of cutaneous sense organs is fully and 

 clearly mapped out in Tremataspis and Bothriolepis. In the former (Fig. 236), 

 each line consists of a series of short, shallow grooves on the smooth outer surface 

 of the shell. A circumorbital line, a.m, postorbital, p.o, occipital oc, a posterior 

 branchial, p.b, and a lateral line p.m, are represented. The occipital lines lead 

 toward the endolymphatic pores described above. No grooves occur on the ven- 



