PARIETAL SENSE-ORGANS OF GEOTRTA. 21 



The latter exhibits a very characteristic crescentic form in 

 transverse section, with the horns of the crescent, which ai-e 

 directly continuous with the wing-like outgrowths of the 

 punctate substance in the anterior enlargement, turned up- 

 wards. The upper part of the crescent is composed chiefly 

 of longitudinal nerve fibres (compare fig. 8) cut across, while 

 the lower part is occupied by a somewhat thinner layer of 

 nerve cells covered by the ependymal epithelium. 



(f) Accessory Structures overlying the Parietal Sense 



Organs. 



The parietal sense organs lie in the cranial cavity imme- 

 diately beneath the connective tissue wall of the cranium, 

 between the nasal and occipital cartilages, as shown in fig. 2. 

 The membranous wall of the cranium (fig. 2, C.T.C.), composed 

 of very dense fibrous connective tissue, thins out somewhat, 

 and is slightly arched upwards in this region, and the upper 

 surfaces of the sense organs are closely pressed against it. 

 Immediately above this there is a thick mass of very much 

 modified connective tissue forming the principal part of the 

 so-called cornea of Studnicka (fig. 2, C.T.P.). This mass of 

 connective tissue is a well-defined structure both in the 

 Lampreys and in Sphenodon, where it occupies the parietal 

 foramen, and it seems desirable to distinguish it by a special 

 name. I therefore propose to call it the " painetal plug.'' 

 In Geotria it consists of a somewhat basin-shaped mass of 

 fibrous tissue, in which the fibres run almost vertically, but 

 converging somewhat below, where the plug is narrower than 

 it is above. The fibres are arranged in dense, multi-nucleate 

 bands, which branch and anastomose freely with one another 

 to form a network with lacunar meshes. Probably these 

 meshes are occupied in life by a gelatinous material, of which 

 traces are still recognisable. The upper ends of the fibrous 

 bands of which the plug is composed are closely attached to 

 the under surface of the corium or dermis. This layer does 

 not appear to undergo any special modification as it passes 



