OF THE COLD-BLOODED VEETEBEATA. 227 



r Post-orbitals and siipra-temporals. Pleurapopbyses long and 

 1 bent. Teeth presenting a complex arrangement of the 



3 < cement Ord. Labteinthodontia. 



No post-orbitals, and supra-temp orals. Pleurapopbyses short 

 V and straight 4. 



. r Peet absent. Body anguiform Ord. Ophiomoepha. 



\ Feet present 5. 



- r Tail persistent Ord. Sozuea. 



\ Tail deciduous Ord. Anuea. 



Class PISCES. 



JBreatliing iy gills only. In some, a sivimming-hladder morpJio- 

 locfically representing, hut not performing tlie function of a true 

 lung. One auricle. No amnion or allantois. Hyoidean and 

 mandibular arches attached as in Amphibia. 



, r "Without jaws 2. 



\ With jaws . 4. 



2 f Heart absent. Suhclassl. Leptocardii, Ord. 1. Amphioxini. 

 I Heart distinct. Subclass 2. Cyclostomi 3. 



3 



Palate perforate Ord. 1. Htpeeoteeti. 



Palate imperforate Ord. 2. Htpeeoaetii. 



' Arterial trunk with a muscular layer and numerous valves. 

 The optic nerves form a chiasma. Intestine furnished with 



a spiral valve 5. 



4<^ Ai-terial trunk •without muscular layer and having but two 

 valves. The optic nerves decussate but do not form a chiasma. 

 No intestine spiral valve. Opercula and free gills. Skeleton 

 perfectly osseous. Subclass 5. Teleostei 9. 



{Gills adherent. No opercula. 

 Subclass 3. Elasmobranchii, s. Selachii 6. 

 Opercula and free gills. Subclass 4. Ganoidei 8. 



{ Branchiae not fixed by their outer margin, with a single aper- 

 ture on each side Ord. 1. Holocepuali, 

 Branchiae adherent by their outer margin, with several aper- 

 tiu-es on each side Ord. 2. Plagiostomi. . 7. 



r Branchial apertures on the neck below. Eyelids connate with 



- J eyes, or absent Subord. 1. Eaiidie. 



] Branchial apertures at sides of neck. Eyelids distinct, with 

 [ free margin Subord. 2. Squalidae. 



n J Skeleton cartilaginous Ord. 1. Chondeostei. 



I Skeleton osseous Ord. 2. Holostei. 



