LINNBAN SOCIETY OF LONDON. 



31 



are powerful cutting structures. Tlie firmour of the head is 

 hinged at each side on the similar bony armour which encircles 

 the anterior part of the trunk. The notochord luust have been 

 persistent, but the closely-set neural and hieiniil arches are 

 superficially calcified. The paired fins seem to have been 

 present, though they are rudimentary. The median fins, of 

 which only one small dorsal and remains of the caudal have been 

 seen, are mere membranes without hard fin-rays. The supports 

 of the dorsal flu are in double series, and directly supported by an 

 equivalent number of neural arches beneath them, as in the 

 Dipnoi and the earliest sharks. 



The condition of the dorsal fin suggests that the Arthrodira 

 are very primitive fishes, and it is interesting to note that they 

 exhibit some other features which confirm this impression. It 

 sufiices to refer to the importance of the pineal organ of the 



Kg. 3. 



Pineal plate of TUanichthys, from the Upper Devonian of Ohio, U.S.A., inner 

 view, one-half nat. size. The paired pineal pits or openings are seen in 

 the middle area of thickened bone. (Brit. Mus. no. P. ^304.) 



brain. In all head-shields in which the impression of this organ 

 has been observed, there is a curious thickening of the surrounding 

 bone, sometimes indeed an excessive thickening. The pit thus 

 formed seems to end blindly, not opening to the exterior by a 

 pineal foramen. Asa rule it is single and must have lodged an 

 ortlinary simple pineal organ ; but in Titaniehthi/s (fig. 3), as now 

 known from three or four specimens, the pit is distinctly paired 

 for the accommodation of a symmetrically paired pineal organ. 

 Prof. Dendy and others have already inferred from the facts of 

 embryology that the pineal must have been originally a paired 

 structure. Tilaniclithys is sufficiently near the beginning of the 

 chordate series actually to show it. 



The skull in the Arthi'odira is very difficult to interpret, 

 because most of the cartilages seem to have remained unossified. 



