962 NEW YORK STATE MUSEUM 



1 The transverse ridges and their lateral extensions running 

 forward to the next transverse ridge were made by the same 

 movement of some organic structure lying transverse to the 

 longitudinal axis of the organism. 



2 The unity, identity and spacing of successive transverse 

 ridges indicate that they were made singly and in succession, 

 that their spacing indicates the forward stride of -the organism, 

 and that it went forward by a crawling, hitching movement. 



3 The varying position of the median ridge with reference to 

 the lateral ridges points to the conclusion that the transverse 

 body was at the time characterized by a mesial sinus or upfold, 

 and that the axis of this fold played to the right and left as a 

 fold might in the flexible muscular foot of a crawling mollusk. 

 In other words, the transverse ridges could not have been pro- 

 duced by the successive ambulatory movements of rigid swim- 

 ming or gill plates, or by the scraping and pressing of the sand 

 through the application to the bottom of the posterior margin 

 of the pygidium of a large trilobite, unless in the latter case it 

 be admitted that the animal was molting and had not grown 

 a new rigid chitinous coat. 



4 The pressed sand on the slopes and over the crest of the 

 transverse ridges eliminates from the processes by which the 

 ridges may have been made the backward push of the posterior 

 margin of such an animal as the trilobite or of any transverse gill 

 plate so placed as not to permit, under the condition in which 

 the creature moved, the smoothing down of the successive 

 ridges by some soft, pliable body still further au derridre. 



An experiment was tried with a thin piece of board cut, as 

 shown in figure 2, to represent the posterior margin of a trilo- 

 bite or the inferior mar- 

 gin of some transverse 

 swimming plate such as 

 the paired gill plates 



Fig 2 Side (a) and end (b) views of wooden block which OCCUr in the ab- 

 used in experiment to produce ladderhke traJs. dominal region of the 



limuloids, with the above restrictions in mind. In order to 

 obtain the medial ridge, it was found necessary to cut the cen- 

 tral sinus shown in figure 2. In this way, by moving the plate 



