MEDIAN EYES IN EXTINCT VERTEBRATES 



341 



the vascular canals contained in the three layers mentioned, with reference 

 to certain polygonal areas and inter-areal grooves which are present in 

 some of the specimens, and small pores which open on the smooth, 



ch 



r.aort 



pr.au aort.gr 



Fig. 238. — Two Schematic Sections through the Head of a Cephalaspid 



Fish. (After Stensio.) 

 A — Median sagittal section. 

 B — Transverse section through the posterior part of the otic region. The exo- 



skeleton shown with thick lines, and the perichondral-bone layers with 



fine lines, cartilage dotted. 



a. eff. com. : space for the arteria efferens 

 communis. 



aort. : aortic canal. 



aort. gr. : aortic groove. 



a. marg. : canal for marginal artery. 



ch. : notochord. 



c. sem. post. : posterior semicircular 

 canal. 



c.v. : cranial cavity. 



d.p.r. : area which bounded the mouth- 

 cavity on the dorsal side. 



d.e.s. : canal for the electric nerve to the 

 dorsal electric field, dsf. 



l.s.f. : lateral electrical field. 



m. : mouth opening. 



na 1 J rna 2 : nasal opening 4- the opening 



of the hypophyseal sac. 

 obr. ch. : oralobranchial chamber. 

 oes.+tr. : opening for the oesophagus 



and trachea + the truncus arteriosus 



in the posterior branchial wall. 

 pin. : pineal foramen. 

 pr. au. : otic prominence. 

 r. aort. : right aorta and aortic ridge. 

 sel. : nerve canal to lateral electrical 



field. 

 vest. : vestibular division of labyrinth 



cavity. 



shining external surface, these appear to have transmitted the external 

 branches from a series of radiating canals originating from a subepidermal 

 vascular plexus. The superficial layer consists of dentine — orthodentine 

 and osteodentine — which, as the dentine canals diminish in diameter as 



