i 2 8 VERTEBRATES WITHOUT JAWS iv. 20 



notochord was persistent and there was no calcined endoskeleton. 

 There were long continuous lateral fin folds and a hypocercal tail. A 

 series of transverse structures were at first interpreted as myotomes 



Pterolepis 



Rhyncholepis 



sec/. 



pec. 



Rhyncholepis 



Figs. 86 and 87. Anaspids seen in dorsal and lateral views. 



an. anal fin; na. nasal aperture; orb. orbit; pec. pectoral spine; pi. pineal foramen; 

 ros. rostrum; sc.d. dorsal scales. (After Stensio and Kiaer and Grasse.) 



but Stensio and Ritchie (i960) consider these to be scales and place 

 Jamoytins with the Anaspida. In either case, the form is of the greatest 

 interest, and represents as White says 'the most primitive of the 

 "vertebrate" series of which we have knowledge'. It is suggested that 

 it might be the ammocoete larva of an ostracoderm (Newth). 



The Heterostraci are actually the oldest known craniates, since their 

 scales occur in the Ordovician. They were common in the Silurian 



