Arthrodires 



595 



the time of founding his " Cirrhostomal Theory" Pollard was 

 unaware of the discovery of Palaospondylus). " Ich muss 

 sagen, das die positive Behauptung der einen wie der anderen 

 Deutung mir sehr unsicher scheint, da auch an den iibrigen 

 Resten des Kopfskelets keine bestimmmten Uebereinstimmungen 

 mit anderen Organismen erweisbar sind. Es ist daher auch 

 nicht zu vermuthen, dass sogar an Beziehung zu Froschlarven 

 gedacht ward. Unter diesen Umstanden mochte ich jene im 

 Verhaltniss zum Kopfe wie zum gesammten Korper bedeutende, 

 von Cirren umstellte Eingangsoffnung als nicht einer Nase, 

 sondern einem Munde oder beiden zugleich angehorig betrachten. 

 Zu einem dem Cyclostomenriechorgan vergleichbaren Ver- 

 halten fehlen alle Bedingungen." 



Relationships of Palaeospondylus. — The arguments for and 

 against the supposition that Palaospondylus is a Cyclostome 

 may be here summed up after Professor Dean, 



The vertebral column agrees with that of the lamprey in 

 having the notochord in part persistent. On the other hand, 

 the vertebra have continuous centra, showing definite processes. 

 Those of the different regions are differentiated. These con- 

 ditions are quite unlike those seen in the lamprey. 



The cranitim is massive, over twice as large proportionally 

 as that of the lamprey. In the latter type the cranium forms 

 but a small portion of the bulk of the head ; in Palaospondylus, 

 on the other hand, the cranium bears every sign of having 

 filled the contour of the head. Moreover, if the region ad- 

 jacent to the structure is admitted to be that of the eye, 

 and few, I believe, will doubt it, then the brain-cavity must, 

 by many analogies, have been much larger than that of a Mar- 

 sipobranch. Also the auditory capsules must have been of 

 extraordinary size. In short, there is very little about the 

 cranium to suggest the structures of Cyclostomes. 



The "oral cirri" suggest somewhat the barbels of the nose 

 and mouth of a hagfish. They, however, resemble even as 

 much in arrangement and greater number the buccal cirri of 

 Amphioxus. On the other hand, similar mouth-surrounding 

 tentacles are evolved independently in many groups of fishes, 

 siluroids, sharks, forms like Pogonias, Hemitripterus. A possi- 



