LXIV BIOLOGICAL SOCIETY OF WASHINGTON. 



with the posterior portion of the skull behind the orbits, and the 

 vertebral column non-segmented or notochordal. 



(2.) The Opistharthri are sharks with the palato-quadrate appa- 

 ratus articulated with the post-orbital processes of the skull, and a 

 persistent notochord, inferior mouth, and the branchial apertures 

 in increased numbers. The Notidanidse or Hexanchidce are the 

 only forms. 



(3.) The Proarthri are sharks with the palato-quadrate apparatus 

 articulated with the anterior orbital region of the skull, the verte- 

 bral column segmented, the mouth sub-terminal, and the forehead 

 declivous. The Heterodontidae, represented by the well-known 

 " Port Jackson Sliark," form the only existing family. 



(4.) The Anavthri are sharks with the palato-quadrate apparatus 

 not articulated directly with the skull, the vertebral column vari- 

 able but more or less segmented, and the mouth inferior. To this 

 group belong all living sharks, excepting those now specifically 

 eliminated. 



(5.) The Rhinai are sharks with the palato-quadrate apparatus 

 also not directly articulated with tlie skull, the vertebral column 

 segmented, the mouth terminal (both jaws being advanced well 

 forwards), and the pectorals developing enlarged anterior basal 

 lobes which are separated by notch-like spaces from the branchial 

 regions. To this belongs the family Squatinidre, including the so- 

 called angel-sharks. 



The speaker was inclined to consider several of these more than 

 sub-ordinal, and rather as of ordinal, value, but until they had been 

 better studied he would reserve opinion on this question. He 

 would now only add that the first and second groups might be com- 

 bined in one order, the third isolated in another, and the fourth 

 and fifth segregated in a third. 



There was also one type represented by the extinct Cladodontids, 

 vv'hose position is doubtful. For these he had formed the group 

 lipospondyli, but it is not evident whether it belongs with the true 

 Squali or whether it may not be related to the Holocephali, the 

 character of the branchial arches being dubious. 



Mr. N. P. Scudder read a paper on Certain Anatomical De- 

 tails IN THE MusKKAT, and exhibited specimens of the skeletons 

 of muskrats, showing the number of the lumbar vertebrce to be six 

 and not three as stated by Profi^ssor Flower. He also showed that 



