1S92.] 



281 



[Cope. 



segmentation, tliebasioccipital element beijg conspicuous. The fins are all 

 primitive, and those of all other types of fishes mighi have been derived 

 from them. Opposed to this estimate of their relation to other vertebrates 

 is the fact that they have not been yet found piior to the Carboniferous 

 period. But our knowledge of the fishes of the Devonian is yet verj'- 

 imperfect. The types ancestral to the Pisces must have existed in the 

 Silurian, and forms which may -well have lulfiUed this lunction have 

 been discovered there. 1 refer to the Agnatha, which have been traced to 

 the summit of the Devonian. The Silurian Agnatha are the Pterasp- 

 rididte, which display the lowest type, and the Cephalaspidida', and these 

 Avere succeeded by the Pterichthyidoe in the Devonian. There is a wide 

 gap between these forms and any of the fishes, and nothing can be 

 affirmed plausibly with regard to the phylogeny. There are superficial 

 resemblances between the dorsal and ventral dermal scuta of the Pterich- 

 thyidse and the Arthrodirous Dipnoi, but there is no considerable affinit}' 

 between those divisions. 



Fig. 2. Xenacantlms dechenii, one of the Ichthyotoiui ; restored by Dr. H. E. Saiivage. 

 From the Coal Measures of France. 



The extinct Agnatha agree with the existing lampreys in the absence of 

 lower jaw and pectoral (scapular) arch, and both must be traced, in accord- 

 ance with Hseckel's phylogeny, to theAcrania, which is now represented 

 by the amphioxus (genus Branchiostoma). This order is easily the ancestor 

 of existing Vertebrata, and shows points of resemblance to both Tunicata 

 and worms. It has been suspected by Dohrn to have undergone degenera- 

 tion, which may have been the case, since this phenomenon is so abun- 

 dantly exhibited by both Tunicata and w^orms. It is not difficult to believe 

 with Kowelewsky, that the Acrania were derived from the Tunicata. 

 Semper has suspected, on the other hand, that the ancestors of the 

 Vertebrata are to be found in the Annelide worms. 



