8 Alexander Petriaikevitch , 



can types. I feel therefore quite sure that the European palaeon- 

 tologists and arachnologists will share with me the feeling of grati- 

 fication at the great opportunity which was afforded me through the 

 kindness of Professor Schuchert, to reexamine and redescribe all 

 American types and to extend the investigation over many as yet 

 undescribed specimens. In view of my indebtedness to Professor 

 Schuchert, at Whose suggestion I undertook the research, through 

 whose exertions the various collections were placed at my disposal, 

 and who throughout my studies has given me many valuable and 

 stimulating suggestions, it affords me great pleasure to dedicate this 

 monograph to him. My thanks are also due to the U. S. National 

 Museum, the University of Chicago, Harvard University, McGill 

 University, the University of Illinois, Mr. L. E. Daniels, and the 

 Peabody Museum of Yale University for permission to reinvestigate 

 their collections. I also acknowledge with thanks the kind per- 

 mission given to me by the Encyclopeadia Britannica Company to 

 reproduce two figures of Liphistius from the eleventh edition of the 

 Encyclopaedia, and by the University Press of Cambridge six figures 

 from Hansen & Sorensen's monograph "On two Orders of Arach- 

 nida." 



Every specimen was carefully cleaned under a binocular micro- 

 scope and the rock covering unexposed parts removed by means of 

 a needle and a small chisel. In some cases the chitin was found still 

 preserved and was examined under higher power. The removal of 

 the white mineral often covering the entire specimen and useful in the 

 detection of appendages concealed in the rock, was not difficult and 

 brought to light many important structures. More difficulties were 

 offered by the rock in which the specimens are imbedded and which 

 had to be removed in some cases to the depth of about V4 of an inch. 

 The specimens which are best preserved are nearly all imbedded in 

 very hard nodules, a clay-iron stone, and required great care in hand- 

 ling. The softer rock surrounding other specimens showed all signs 

 of decomposition and here the chiseling was easier but the preser- 

 vation of the specimens themselves, not so good. After careful study 

 under the binocular with the aid of artificial light wliich allows a 

 more perfect reflection of the rays from the exposed surfaces, a draw- 

 ing of each specimen was made by measuring its parts and enlarging 

 them a given number of times. Thus all the text figures represent 

 the specimens more or less correctly as they appear under the micro- 

 scope. In many cases, however, both halves of the concretion show 

 superimposed imprints of the dorsal and ventral surfaces of the spe- 

 cimen and obscure the actual structure. In such cases the super- 



