194 J- A. UDDEN 



apparently a minute pteropod. I have, however, been unable to 

 find any spiral or helical forms like that of Spirophyton typum. 

 The nearest approach to it is a twisting of that end of the cake 

 toward which the concave sides of the ridges are directed 



There are several circumstances which point to a mechanical 

 origin of these structures. The material of which they consist is 

 a compact calcareous mass apparently identical to that of the sur- 

 rounding rock. It is difficult to explain how this could have 

 been introduced in such quantity and in such condition into the 

 interior of a soft pulpy seaweed, and still have permitted the 

 plant to leave a mold of both of its surfaces in the accumulating 

 sediments. Another circumstance of similar significance is the 

 indistinctness of the margins of some of the specimens. In their 

 shapes, moreover, there is an indefinable lack of that uniformity 

 of design which we are accustomed to find in organic forms. 

 Unless certain ones are selected and others left out, classification 

 on this basis seems impracticable. From Hall's description of 

 the spiral-growing fucoids in the Devonian rocks in New York, 

 it is evident that these also are variable in form, for he states 

 that " the larger fronds not unfrequently present irregularities 

 and distortions, both from unequal growth and from accident, 

 evidently having been very flexible and easily disturbed," and 

 he refers to some of the specimens as being "detached portions 

 which have been distorted by pressure after their separation." 



EXPLANATION OF PLATE VII. 



Fig. i. Dimensions: length, I9 cm ; breadth 5.3°™; average thickness, 2. 5 mm . 



Fig. 2. Dimensions: length, I2.5 cm ; breadth 8 cm at the widest point; average 

 thickness, 2 mm . 



Fig. 3. Dimensions: greatest diameter, I3 cm ; average thickness, 4 mm . Evi- 

 dently fragmentary. 



Fig. 4. Dimensions: length, I2 cm ; greatest breadth, io.5 cm ; thickness from 

 6 mm to 2 mm . 



Fig. 5. Dimensions: length, i6 cm ; average breadth, 5 cm ; average thickness, 

 4-5 mm - 



Fig. 6. Dimensions : greatest diameter, I4 cm ; average thickness, 4.5 mm - Frag- 

 mentary. 



