950 PRINCIPLES OF STRATIGRAPHY 



feet or ambulacra, and five interambulacral. The whole forms a 

 more or less solid corona. In the post-Palaeozoic types each zone is 

 generally composed of two columns of plates, so that there are in 

 all 20 columns, forming 5 ambulacral and 5 interambulacral zones. 

 The mouth and anus are generally opposite each other in the 

 Pahvechinoidea, and in the Cidaroidca and Diadeinatoidea. In the 

 others the anus migrates toward the mouth. The Clypcasteroidea 

 and Spatangoidca show an elongation of form, with a pronounced 

 bilateral symmetry. In most of the Spatangoidca the mouth passes 

 forward, so as to lie no longer in the median axis. In the Holo- 

 thuroidca- the plates of the integument are not united, the body 

 thus being soft and changeable in form by inflation. The separate 

 plates are found fossil as early as the Carbonic. 



Phylum X — Protochorda. These are soft-bodied animals, 

 some of them, as the Tunicates, degenerate, but showing affinities 

 with the vertebrates, in the possession of a notochord, branchial 

 slits, and a central nervous system. The Cephalochorda (Am- 

 phioxus) are fish-like and readily mistaken for a vertebrate, while 

 the Enteropnciista (Balanoglossus) are worm-like. While some of 

 these have been considered ancestral to vertebrates, it is not at all 

 impossible that the suggestive characters are independently de- 

 veloped. Vertebrates arose in the Palaeozoic, and no modern form 

 is likely to preserve intact all the primitive characters of a class. 



Phylum XI — ^"ERTEBRATA. This, the most highly specialized 

 phylum of the animal kingdom, has its most primitive representa- 

 tive in the Ostracoderma of the early Palaeozoic (Cephalaspis, 

 Pterichthys, Bothriolepis, etc.). Known definitely from the De- 

 vonic and Siluric, there are fragments indicating their existence 

 in the Upper Ordovicic of America. They retain many characters 

 of invertebrates and seem to unite the fish with the eurypterids, a 

 group of Merostomes, which flourished at the same time. (See 

 Patten-2i ; 22.) Their most striking characteristic was a well- 

 developed armor, or exoskeleton of bony plates, which covered the 

 head and anterior portion of the body. The endoskeleton was not 

 calcified and the mouth without hard parts. Hence all we know 

 of them is from the external plates and scales. The Devonic 

 Artlirodira have also been regarded as an independent class, dififer- 

 ing from fishes in that their jaw elements are merely dermal ossifi- 

 cations and are not articulated with the skull (Dean). The head 

 and trunk are covered by symmetric bony plates, the head-shield 

 is movably articulated with the body-shield. The endoskeleton is 

 superficially calcified, and paired fins are rudimentary or absent. 

 The Devonic Coccosteus, Dinichthys and Titanichthys are examples. 



