484 



GEOLOGY. 



or lower jaws of the vertebrate type. On the contrary each jaw con- 

 sists of two separate parts which act upon one another, not verti- 

 cally, in vertebrate fashion, but laterally, as in arthropods. This is 

 especially true of the upper jaw. While perhaps they should remain 

 under the broad mantle of the term Chordata, they must apparently 

 be removed from the true vertebrates to a new class. 



The forms which show most strikingly the arthropodian anterior 

 and the vertebrate-like posterior, are the cephalaspids (head-shield) 

 illustrated in Figs. 213 and 214, restorations by Patten. The broad 



shovel-like head, with the 

 eyes close together on the cen- 

 tral ridge, is interpreted as an 

 adaptation to plowing in the 

 mud, half-buried, after the 

 fashion of the modern king- 

 crab (Limulus), to which the 

 cephalaspids are believed to 

 have borne more than a super- 

 ficial resemblance. The eyes 

 were protected by a thin coat- 

 ing of hard material continu- 

 ous with the outer layer of the 

 shield. The large, segmentally 

 arranged trunk-scales, the tail 

 and the fins, are decidedly 

 fish-like, while the projected 

 side angles of the shield 

 (cornua), so trilobite-like in 

 aspect, as well as the cephalic 

 appendages, are curious fea- 

 tures that have awakened 

 much discussion; as have also 

 the fringing appendages along 

 the side of the body. 



The Pteraspis, the earliest 

 of these curious creatures to 

 appear in America (Salina), is still imperfectly known and the subject 

 of pronounced differences of interpretation. The Cyathaspis, Fig. 215, 



Fig, 216. — Reconstruction of the head and trunk 

 of Tremataspis , seen from above. Natural 

 , size. (After Patten.) 



