NOTICE. 



nnilE third and fourth Parts of the Eevision -were to have been issued 

 -*- simultaneously. Owing, however, to the destruction, in the great con- 

 flagration of November 9, 1872, of the lithographic stones of six of the 

 Plates of Part IV., together with the original drawings, and all the MS. 

 Notes on the subject, Part III. is issued in advance, with the Plates 

 belonging to Part IV., in order not to delay the publication of the mere 

 descriptive Part (Part III.). 



It was impracticable to refer in Part III. to publications which have 

 appeared since the issue of Parts I. and II. not included in the Bibli- 

 ography and Synonymy, as the greater portion of the descriptions of Part 

 III. has been in type for some time. 



In Part III. no reference has been made in some of the specific descrip- 

 tions to a few plates which were not struck off at the time the descriptions 

 were printed. This is not of great importance, as the plates referred to 

 were intended to illustrate general points of structure. A complete Index 

 of the contents of the Plates is given to obviate this defect. 



ALEXANDER AGASSIZ. 

 Cambridge, September, 1873. 



