EXPLANATION OF PLATE IL 



Triarthrus Becki Green. 

 Page 23. 



Fig. 1. Natural size and enlargement of an individual having one thoracic 

 segment. 



Fig. 1 a. A narrow and more elongate individual of the first known degree of 

 development having one segment in the thorax. 



Fig, 15. Natural size, and enlargement of 1 a, to fifteen diameters to show the 

 character of the head and pygidium and their relative proportions 

 and size. 



Figs. 1-13. A series of individuals illustrating the gradual development of the 

 head and throas on the addition of each thoracic segment The 

 pygidium diminishing in size as compared with the other parts 

 of the body. The numbers 1-13 also indicate the number of 

 segments in the thorax of each individual to which they refer. 

 All enlarged to three diameters. 



Fig. 13. Enlargement to three diameters of an individual having fourteen 

 thoracic segments. 



Fig. 14. Fully developed individual of sixteen thoracic segments, natural size. 



All the larger specimens have been flattened by compression. The 

 csnvexity in the figure is the same as in an individual of sixteen 

 segments, 33™™ in length. 

 . : The free cheeks are also pressed out so as to show their margins. 



Cyathophycus heticulatus. 

 Page 18. 



Fig. 16. A frond presenting the usual characters when flattened between the 



layers of shale. 

 Fig. IG a. A small frond showing the true cup like form. 



Fig. 16 b. A more elongate tubular frond with a somewhat contracted aperture. 

 Fig. 16 c. A fragment of a frond showing the thickened cell walls. 

 Fig. 16 d. Base of a frond flattened down upon the frond. The last is from Mr 

 E. Hurlburt's collection. 



Cyathophycus subsphericus. 

 Page 19. 



Fig. 17. A flattened frond showing the plain structure and the small opening 

 at the summit. A portion of the margin is broken away. 



DiSCOPHYCUS TYPICALIS 

 Page J9. 



Fig. IH. A small characteristic specimen. The margin is slightly corrugated 

 and numerous individuals of Ortliu testudinaria are attached to 

 the surface. 



