56 



IOWA GEOLOGICAL SURVEY 



Fig. 3. 



Fig. 3. Ateleaspis tesselata Tract. Silurian (Downtonian) of Scotland, 

 restored outline showing tail in profile, x k (after Traquair). 



Diagrammatic 



by complete skeletons in which the dermal covering consists 

 merely of small shagreen tubercles, sometimes beautifully orna- 

 mented, or of minute hollow spines. Another interesting form 

 of great systematic importance is Ateleaspis (Fig. 3), which is 

 regarded by Dr. Traquair as annectant between Heterostraci and 

 the next higher order, Osteostraci. Under the last-named are 

 mcluded such well known forms as Cephalaspis (Fig. 4, page 58), 

 Tremataspis, Thyestes, etc. Only in the case of the most spe- 

 cialized order, Antiarchi, comprising the single family Aster- 

 olepidae, are we unable to trace the lines of descent, owing to non- 

 acquaintance with intermediate forms linking this group with any 

 of the preceding. The suggestion that Asterolepids may have 



