352 JOHN WARREN 



arch. This segment of the diencephahc roof varies greatly in 

 its development. In almost all forms except some elasmobranchs 

 it develops dorsally to a greater or lesser degree into a dome- 

 like expansion which is very striking. The roof is always a thin 

 membrane, consisting of a single layer of low cells. In many- 

 cases it becomes very convoluted and forms vascular tufts which 

 represent the diencephalic plexus. This region has been called 

 by various names, 'Dorsal sack,' Goronowitsch, '88, 'Zirbel- 

 polster,' Burckhardt (11), 'Parencephalon,' Kupffer (57), 

 'Postparaphysis,' Sorenson (90). I have used chiefly the term 

 post velar arch though in the later stages 'dorsal sack' might 

 be more appropriate. In Petromyzon Burckhardt (13), pi. 8, 

 there is practically no velum. The hinder border of the mouth of 

 the paraphysis is a mere lip and extending caudad from it is a 

 short, low post velar arch. In Acanthias the velum is seen early 

 as a sharp angle separating the paraphysal arch from the post 

 velar arch Minot (71) figs. 1 and 2. It develops rapidly in a ven- 

 tral direction absorbing in this process all the post velar arch so 

 that the superior commissure and the opening of the epiphysis 

 come to lie close against its base. Both walls of the velum espe- 

 cially the cephalic become convoluted later to form the dienceph- 

 alic plexus which hangs down like a curtain across the cavity of 

 the diencephalon, Minot (71), figs. 1-12. D'Erchia (24) shows in 

 Pristiurus much the same condition, figs. 1-6, and also in Torpedo, 

 figs. 7-15. Here, however, the post velar arch persists to some 

 extent. 



In Accipeliser, Kupffer (57), figs. 117, Burckhardt (13), pi. 8, 

 there is a long velum the cephalic wall of which is convoluted, 

 the caudal being perfectly flat. It resembles closely the velum of 

 Acanthias, but the post velar arch forms a high vaulted dorsal 

 sack so that the superior commissure is here separated by a long 

 interval from the velum. In Protopterus Burckhardt (9) finds 

 essentially the same condition as in Accipenser. In Amia and 

 Lepidosteus Kingsbury (54) describes lateral expansions from the 

 dorsal sack. On either side diverticuli pass out just behind the 

 velum. These divide into a cephalic limb extending forward to 

 the olfactory lobes and a caudal limb extending backwards to 



