2o8 ROCHESTER ACADEMY OF SCIENCE. 



branch, and it is not unusual to find a number of them closely approxi- 

 mated and radiating from their intercepting ends. The casts can be 

 separated, showing one continuous through the other. The cuttings 

 afford the means of determining the relative ages of the structures, for 

 the one cut had to exist before the other could cut it. 



D. archimedes occurs in the sandstone and sometimes in the more 

 arenaceous partings. The flat forms stand vertically in the rock, the 

 rounded end down (see Figure i). They are frequendy 12 to 14 

 inches in depth, and at the same time rarely exceed 4 inches in 

 breadth and ^ inch in thickness. Both faces are crossed by a series 

 of sagging ridges ascending higher at one end than the other. The 

 edge at which the upper ends terminate is the thicker. The ridges 

 are the edges of curved strips, lunate in section, which saddle one 

 upon the other. At their upper ends they thin out and lap upon a 

 marginal cylinder so as to partially enclose it. The cylinder extends 

 around the base, terminating in a blunt point at the thin edge. At 

 this edge the end of each itrip forms a conical cap which partially 

 ensheathes the one beneath, the lowest capping the tip of the cylinder. 

 In many specimens traces of corrugations like those of Arthrophycus 

 occur upon the cylinder and the edges of the strips. Sometimes they 

 are so well defined as to impart a crenulated appearance to the surface 

 of the plate. They do not, however, occur upon the contiguous 

 surfaces. Where a base projects into an underlying shale, it may 

 show even the fine wrinkles. 



Although many of the plates are practically flat, others are 

 crimped and contorted into an infinite variety of shapes. Many 

 specimens which are flat in the upper portion, have in the lower, the 

 form of an inverted archimedean spiral (see Figure 2) , either dextral 

 or sinistral, each volution having from the side the appearance of an 

 oblique cone with its apex in the cavity of the one above. The 

 margin of the spiral is formed by the thin edge. At the base the 

 cylinder appears as the continuation of this edge, rounds upward, and 

 passes through the center of the structure. The curving strips, 

 which in the flat plates have a general transverse direction, turn 

 upward from the margin. 



As in ArthropJiyais, penetration of one structure by another is of 

 constant occurrence. In the spirals the cutting of one portion by 

 another portion is also very common. One specimen preserving ^}^ 



