CLASS AGNATHA. 



.Sub- Classes* Orders. 



A. Hyperoartia. 



1. Cyclostomi. -?2. Hyperotreti. 



UJ. .Cycliae. 



fl. Heterostraci. 



2. Ostracodenni. < 2. Osteostraci. 



( 3. Antiarchi. 



IT is generally admitted that the existing Cyclostoraes or 

 Marsipobranchs are the degenerate representatives of a Class 

 which must have attained a much more extensive development 

 at some remote period ; but the only known fossils as yet 

 capable of being referred to such a Class have been obtained 

 from the Upper Silurian and the Devonian rocks. Even 

 these are quite of a problematical character; but as none of 

 them have hitherto exhibited certain traces either of a lower 

 jaw or paired limbs, they may be conveniently ascribed to this 

 lowly group for the present. One of them seems to be clearly 

 monorhinal, though with a calcined endoskeleton ; all the 

 others possess a strong dermal armour, without any calcification 

 of the endoskeletal parts. It is probable that no member 

 of the Class with hard structures capable of fossilization has 

 existed since Palaeozoic times.. 



Sub-Class 1. Cyclostomi. 



The minute denticles known as Conodonts, met with in 



Palaeozoic rocks from the Lower Silurian to the Carboniferous 



Limestone inclusive, are frequently compared with the teeth 



.of the modern lampreys and hag-fishes. Their histological 



structure, however; is so different from that of any teeth 



known, that their affinities are quite indeterminable. They are 



w. 1 



