THE PLAGIOSTOMIA. 



(SHARKS, SKATES, AND RAYS.) 



INTRODUCTION. 



The purposes of this Memoir are to make additions to knowledge of the 

 Sharks, Skates, and Rays (Plagiostomia) and to present the subject in such a 

 manner as will be most useful to the greatest number of students. If in further- 

 ance of these intentions it is found that there has been some elimination or 

 apparent neglect of particular special advances in the science it is to be remem- 

 bered that the latter being technical appeal more directly to the few specialists 

 than to the much greater number of general inquirers. The work is necessarily 

 a revision; and being a revision, however original, that begins at the same 

 sources, and treats of the same material, literary or other, as a classical work 

 like that of MuUer and Henle, 1841, it must of necessity be to a great extent 

 confirmatory in its results; and it must either begin at the same sources and go 

 over the grounds traversed by the earlier authors or it must accept their con- 

 clusions. Going over the entire field as is done here discovers many occasions 

 for difference of opinion. This is most true in the nomenclature and synonymy. 

 Happily there has been a somewhat general agreement among zoologists in 

 regard to nomenclature of animal species that it is not advisable to go farther 

 back, or to earher dates, than 1758, the date of the tenth edition of the ^'Systema 

 naturae" of Linne; but unhappily a few writers prefer to select their authorities, 

 later than Linne, as binomial or nonbinomial rather than to accept the earliest, 

 after 1758, properly formed and bestowed binomial names by whomsoever made 

 and applied. The selection of one authority because he favored binomials more 

 than another has led to much uncertainty among names and to many changes. 

 It has led authors to belittle and ignore excellent works which at their time of 

 pubhcation and much later ranked in accuracy and influence among the first of 

 the scientific publications of their period. In support of this, two works to 



