MEDIAN EYES IN EXTINCT VERTEBRATES 347 



of a bony skeleton, although it is probable that a cartilaginous endo- 

 skeleton did in reality exist, but it has left no evidence of its presence, 

 the cartilage not having been fully calcified and thus not being preserved. 

 The strata, according to Kaier, appear to have been laid down rapidly 

 at the estuaries of large rivers which in periods of floods deposited thick 

 layers of mud, in which the fossils were imbedded and preserved in situ. 

 The remains are thus in some cases remarkably complete and perfect, 

 and although sometimes distorted, it is possible to reconstruct the parts 

 and make comparisons of the different species, as shown in Fig. 244, in 

 which it will be seen that a series or branchial apertures are present 

 which resemble those in living species of Marsipobranchia or cyclostomes ; 

 and also that pectoral spines were present which it is believed correspond 

 to the pectoral fins of fishes. Moreover, since the olfactory organs of the 

 cyclostomes, although opening on the surface by a single aperture, are, 



Fig. 244. — Reconstruction of Pterolepis nitidus. (After Kiaer.) Down- 

 tonian strata. rlngerike, showing orbital scleral plates, ten 

 branchial apertures, rudimentary pectoral fin, and hypocercal tail. 



as was strongly emphasized by Gaskell, truly bilateral in nature — there 

 being paired olfactory nerves and paired olfactory lobes of the brain both 

 in the living and extinct species of Cyclostomata — and since there is a 

 close correspondence in the brain, sense-organs, cranial nerves, and many 

 other structural points between the cyclostomes and fishes, it is obvious 

 that they must have originated from a common stock ; and the circum- 

 stance that the existence of a single nasal aperture in the class Cyclosto- 

 mata has been employed as the basis of their classification as a separate 

 branch — " Monorhina " — must not be thought to imply that they are 

 totally different and have nothing to do with the class Pisces. Further, 

 the presence of a pineal foramen located in the centre of a pineal plate 

 which is situated between the orbital cavities and the proof of the bilateral 

 nature of both the pineal and olfactory organs constitute along with other 

 structural resemblances important evidence substantiating this conclusion. 



Geological Evidence of the existence of Median Eyes in Invertebrates 



In tracing the evolution of median eyes from the earliest known 

 classes of fossil animals which show indications of such, we may consider 



