4 FREDERICK W. SARDESON 



structure be discovered. The use of thin sections is, however, 

 necessarily limited to special cases. These would be when a 

 new species is in hand and all characters possible should be dis- 

 covered ; or, when a described species is illustrated and described 

 chiefly as to its internal structure, which too frequently is the 

 case; or, when the external characters have been obliterated. 

 Having identified the species and referred it to a genus, etc., by 

 use of all characters, further recognition of specimens of the 

 same can and should, with rare exception, be made to depend 

 on external characters alone. As in studying Brachiopoda, for 

 example, one must know them by external characters, even 

 though examination of internal structures is required to deter- 

 mine affinities of the species. 



The advantage of learning to recognize the species, genera, 

 etc., by external characters is in«the saving of time, since thou- 

 sands can be examined in that way, while sectioning limits the 

 labor of one man to at most twenty specimens per day ; knowl- 

 edge of the range and variability of one and many species is made 

 practicable ; it serves to direct to best advantage the use of thin 

 sectioning; recognition of species even in the field becomes 

 thereby entirely practicable. Having learned to know a group of 

 species, or the fauna of a locality or of a zone, the specimens 

 may be identified thereafter without the use of sectioning, and 

 the difficulty of technique may be obviated by the geologist. 



Thin sections of fossils may be made by the same process 

 as bone or rock sections are ground, which need not be described 

 here. It requires less skill, however, since they should not be 

 ground to absolute thinness. Some simple appliance for meas- 

 uring the cell dimensions is also needed. 



Pains may be saved by attention in collecting, since each bed 

 or zone may have a large proportion of species peculiar to it, 

 and by avoiding mixing fossils of different zones, labor of again 

 assorting is saved. 



TREPOSTOMATA 



A few selected species may illustrate what is to be looked 

 for in Monticuliporoidea. Beginning with Trepostomata one 



