342 AXXALS XEW YORE ACADEMY OF SCIENCES 



and of the shell-covered flippers of tortoises. A few authors have even 

 endeavored to homologize the paired appendages of Antiarchi with the 

 pectoral limbs of gnathostomes ! Pelvic paired fins are absent in the 

 Ostracodermi and Antiarchi, and also in the Arthrodira, as Dean (1909, 

 pp. 282-287) has shown. 



ARTHRODIRA 



The exoskeleton of the Antiarchi afforded an ample base for their 

 ]:taired appendages as well as for the muscles of the head and thorax, and 

 the same is true in the case of those Arthrodira which, like Acanihaspis, 

 ]jad paired spine-like appendages attached to the anteroex;ernal corners 

 of their osseous plastron. Here it may be noted that the attempts of 

 Jaekel, Tate Eegan and Patten to homologize the elements of the thoracic 

 plates of Arthrodira with the dermal plates of the shoulder-girdle of 

 Osteichthyes appear to the present writer to constitute a begging of the 

 question. There are, it is true, vague resemblances between the "clavicu- 

 lar" and other thoracic plates of Arthrodira and the pectoral plates of 

 Osteichthyes, but in view of the amazing powers of convergent evolution, 

 which are known by experience to many investigators, why sliould we 

 assume a homology or a series of homologies and then regard them as 

 a basis for phylogenetic speculation? The ^Tiomologies" assumed by 

 Jaekel, Patten and Tate Eegan are based merely upon a general similarity 

 in the spatial relations of certain plates in Arthrodira and Osteichthyes 

 with reference to assumedly homologous starting-points. Given paired 

 orbits in the two phyla and Jaekel will call the median dorsal element 

 lying between the orbits *^frontale," the elements behind it "parietalia'' 

 and the median occipital element '^occipitale superius'^ and assume that 

 the so-named plates are homologous with those of Osteichthyes. And the 

 supposed homologies of the arthrodiran shoulder plates with those of 

 Osteichthyes are equally arbitrary and unconvincing. This matter is of 

 some importance to our main topic, the origin of the Tetrapoda, for in 

 Professor JaekeFs early schemes the Tetrapoda and the Placodermi were 

 indicated as having sprung from a common pro-tetrapod stock : Patten 

 too connects the Arthrodira with the stem of the Dipnoi and Amphibia 

 and homologizes the pectoral plates of Dipnoi with those of Arthrodira. 

 According to the view here adopted the Arthrodira may be an offshoot 

 from the antiarchian stem, which "paralleled'' and even surpassed the 

 Osteichthyes in the development of the exoskeleton of the head and 

 thorax, but failed to build up tlie endoskeleton to the same degree. Xor 

 were their accessory locomotive organs (caudal fin, dorsal fin) as highly 

 crganized as in either the Elasmobranchii or the Osteichthyes. 



