1889.1 Synopsis of the Families of Vertebrata. 853 
Orbits separated by a plate which is pierced by two 
orifices, possibly nostrils ; Diplorrhina. 
To the Arrhina belong the families of the Pteraspididae and 
Cephalaspidae, and to the Diplorrhina that of the Mycteropidae. 
The latter has some affinity to the Pterichthyidae, which has 
similarly approximated orbits, or orifices which correspond with 
those provisionally termed orbits in Mycterops. They are sepa- 
rated in the Pterichythidse by a movable plate which is not perfor- 
ated. Pterichythida; differ in the possession of a peculiar dorsal 
shield, which resembles that of the Tunicate germs Chelyosoma. 
The tail, present in Pterichthys, is wanting in Bothriolepis, calling 
to mind the absorption of the tail in the Tunicates. The lateral 
appendages resemble, those of the Tunicate genus Appendicu- 
laria. F'or the above reasons I hav^e placed tNe Pterichyidae in 
the Tunicata, as the type of a distinct order, the Antiarcha. 
This order may belong to the Ostracodermi. 
MARSIPOBRANCHII. 
This subclass has two orders. 
Branchial fissures communicating directly with the 
pharynx ; nasal sac perforating the palate ; Hyperotreti. 
Branchial fissures communicating with a common 
branchial passage which opens into the pharynx ; 
nasal sac not perforating palate ; Hyperoarti. 
To the Hyperotreti belong two families, the Myxinidae and 
the Bdellostomidae ; to the Hyperoarti one, the Petromyzontidae. 
PISCES, 
This class is divided into four subclasses. 
I. No suspensorium of the mandible. 
No dermal cranial ossifications ; ventral claspers ; 
no opercular bones ; no maxillary arch ; Holocephali. 
Dermal cranial ossifications and opercular bones ; 
no claspers ; no maxillary arch ; Dipnoi. 
II. A suspensorium of the mandible. 
No dermal cranial ossifications nor opercular 
bones ; claspers present ; Elasmobranchu. 
