EDWARD D. COPE. 63 



The history of the Tunicata cannot be traced by the 

 paleontologist as yet, owing to the absence of hard parts 

 in their structure. The evidence of embryology lias. 

 however, convinced phylogenists that the ancestors of 

 this class resemble their larvae, and that they have as a 

 whole undergone a remarkable degeneracy. They have 

 passed from an active, free life to a sessile one, and have 

 lost the characters which pertain to the life of verte- 

 brates generally. 



It was to have been anticipated, however, that all of 

 these ancestral Tunicata did not undergo this degenera- 

 tive metamorphosis, for it is to such types that we must 

 look for the ancestors of the other Vertebrata, the Acra- 

 nia and the Craniata. And here paleontology steps in 

 and throws new light on the question. I have pointed 

 out briefly, in the American Naturalist, 1 that a second 

 order must be added to the Urochorda, viz., the Anti- 

 archa, in which the anus presents the same position 

 as in the Aerania, at the posterior end of the body, while 

 an orifice of the upper surface represents the mouth of 

 the Tunicata. To this order is to be referred the family 

 of the Pterichithyidse, of which the typical genus, Pter- 

 ichtys, is a well-known form of the Devonian period. 

 This genus retained its tail, which was the cause, in con- 

 nection with the presence of lateral fin-like appendages, 

 of its having been supiDosed to be a fish, by Agassiz, 

 Hugh Miller and others. It is possible that the American 

 Bothriolepis canadensis lost its tail, as in the majority of 

 Urochorda. The tunicate which approaches nearest to 

 the Antiarcha is the arctic Chelyosoma. 



From the Antiarcha to the Aerania and Craniata, then, 

 the line is an ascending one. 



II. THE L1XE OF THE PISCES. 



The fishes form various series and subseries, and the 



1 March number, 1885, under " Geology anil Paleontology." 



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