196 J. A. UDDEN 



The general appearance of the western specimens is such as to 

 suggest that they have been formed from flowing mud. In some 

 instances the crescentic ridges overlap, as if the flow had run 

 over on itself. Near the edges of the specimens planes of divi- 

 sions are sometimes seen, which readily might be accounted for 

 as planes of differential motion in moving mud, but which seem 

 difficult to explain if the specimens be regarded as imprints or 

 casts of sea-weeds. 



But the resemblance between these western specimens and 

 Spirophytoji is in general so perfect that there can hardly be any 

 doubt that both have the same origin, and it is evident that the 

 regular spiral frond of the latter, however rarely found, cannot 

 have been produced except by the intervention of some organic 

 agency. The quiet sea-bottom indicated by the nature of the 

 sediments containing the fossils also preclude the possibility of 

 the occurrence of mud-flow in the ordinary way under the con- 

 ditions of deposition of the containing rock. 



It seems, however, that cakes like these might be formed in 

 just such situations from the voided contents of mud-eating ani- 

 mals, such as sea-cucumbers. These are known to burrow in the 

 mud, far out in the sea, and to extract their food from such frag- 

 ments of organic substance as they find in the ooze, mud, and 

 sand with which they fill themselves, and which is afterward 

 expelled by contractions of the visceral muscles. This may, no 

 doubt, take place down in the mud as well as on the surface of 

 the sea-bottom. If we suppose that the cakes have been formed 

 in such a way, it is easy to account for their diversity of form — 

 even when this takes on the complex twist of a spiral — and 

 there will be no difficulty in explaining how it came about that 

 they are molded from the same material as the surrounding 

 matrix. A loose mud slowly forced out from a receding tube 



EXPLANATION OF PLATE VIII. 



Fig. 7. A specimen in the matrix. Length, I2 cm ; breadth, io cm ; thickness, 2 mm . 



Fig. 8. Fragment of a large specimen cut by a joint in the rock obliquely, and 

 confluent at the edge with a layer in the rock. Greatest length, 2i.5 cm ; breadth, 

 I4 cm ; average thickness, 4.5 mm . 



