ANALOGIES. SQUALINCE AND CENTRING. l63 



respectively placed in each column. We must first, 

 however, remind the naturalist, that the groups in these 

 columns are what we consider to be genera; and that, 

 consequently, those numerous sub-genera which have 

 been proposed by other naturalists, and whose names do 

 not appear in the above list, are considered by us of 

 subordinate rank, and form a part of one or other of 

 these genera. One instance will better explain our 

 meaning : M. Cuvier's sub-genus Selache is not men- 

 tioned,, because we consider Rafinesque's Isurus is the 

 true typical example of the genus which connects the 

 Squalince and the Centrince. This union of two circles 

 is always effected either by the rasorial or the fissirostral 

 type ; and the long snout and forked tail of Isurus 

 clearly show that it is of this latter description : 

 Selache, indeed;, is an excellent sub-genus, but it is sub- 

 ordinate to Isurus j and therefore, as every group should 

 bear the name of its most typical example, we name it, 

 in the present instance, accordingly. In like manner, 

 PrUtiurus of Bonaparte, from what little has been said 

 of its form*, appears the true type of Scyllium, because 

 it is said to have a "long snout," — the exact character of 

 Isurus in the opposite circle of the Squalince £ so that 

 we may fairly suppose they are analogous. As our 

 acquaintance, however, with Pristiurus is so slight, we 

 shall for the present retain the long-employed name of 

 Scyllium. But it may fairly be asked, Upon what prin- 

 ciple do we ground our belief that one group is a genus, 

 and that another is a sub-genus ? and why, in reference 

 to the above case, has not Selache as great a claim to be 

 considered the type of a genus as Isurus ? To this 

 we reply, by stating the especial object of the present 

 inquiry: our purpose is to show that each of the two 

 typical sub-families of the sharks — the Squalince and 

 the Centring — represent each other in their respective 

 circles ; and that the subordinate divisions, or genera, 



* Muller and Henle, Mag. of Nat. Historv. 2d series, voL ii. p. 3£. 

 M 2 



