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genus ( Ceratodus ) testify to its presence ; no further trace of it 
has been found until the present period, where it reappears in 
three genera ( Ceratodus , Lepidosiren , Protopterus ), one of 
which is identical with that of the Mesozoic era. Now, at 
present scarcely any zoologist will deny that there must have 
been a continuity of the Dipnoous type, and it is only a proof 
of the incompleteness of the palaeontological record that we 
have to derive all our information regarding it from only three 
so very distant periods of its existence. 
In conclusion I may add a synoptical table, from which the 
systematic views advocated above, and more especially the posi- 
tion of Ceratodus in the system, may be readily understood. 
After the separation of Amphiosus and the Lampreys as types 
of two distinct sub-classes ( Leptocardii and Cyclostomata ), 
the remaining host of fishes are referred to two other sub- 
classes : — 
Stjb-class : Teleostei. Heart with a rigid hulbus aortse ; intestine without 
spiral valve ; optic nerves decussating (living species, nearly 9,000). 
Sub-class : Palceichthyes. Heart with a contractile conus arteriosus ; intes- 
tine with a spiral valve ; optic nerves non-decussating. 
Order I . — Playiostomata, or Marine Palceichthyes (sharks and rays ) living 
species, nearly 300). 
Order II. — Holocephala (four living species). 
Order III. — Ganoidei, or Freshwater Palceichthyes. 
Sub-order 1. — Amioidei (one species). 
Sub-order 2. — Lepidosteoidei (three species). 
Sub-order 3. — Polypteroidei (two species). 
Sub-order 4. — Chondrostei (sturgeons, thirty species). 
Sub-order 5. — Dipnoi. 
Fam. a. — Sirenidce. 
Sub-fam. — Ceratodontina (Ceratodus). 
Sub-fam. — Protoptei'ina (Lepidosiren, Protopterus). 
Fam. b. — Ctenododipteridce (Ctenodus, Dipterus). 
Fam. c. — Phaneropleuridce (Phaneropleuron). 
