VERTEBEATA 



No osseous skeleton nor limb-like append- 

 ages; 



15 



Marsipobranch ii. 



Of the Agnatha, extinct species of the subclasses Ostraco- 

 phori and C3'clia3 only are known. 



Subclass I.— OSTRACOPHORI. 



There are three orders of this subclass, as follows. In 

 none of them are nostrils present : 



No pectoral appendages; exoskeleton in three 

 layers, — the internal lamellar, the middle 

 cancellous, the external vaso-dentine ; 

 no bone-corpuscles; Heterostraca. 



No pectoral appendages; exoskeleton in three 

 layers with bone-corpuscles, the middle 

 layer with vascular canals; Osteostraca. 



Pectoral appendages; exoskeleton osseous; 



with sensory grooves; Antiarcha. 



The time of appearance and range in time of these or- 

 ders is displayed in the following table. They are all 

 Paleozoic. 



