Notes on Octrhoniferous Selachii. 415 



shoulder girdle, and evidently represent the propterygium and 

 metapterygium of ordinary Selachii. Behind these is an 

 oblong metapterygium, bearing radials preaxially, whose an- 

 terior portion seems to have absorbed the bases of one or two 

 adjacent radials, but whose posterior extremity is continued 

 backwards as a long narrow segmented stem consisting of 

 nine rectangular joints, and reminding one at first sight of a 

 vertebral column ! This part in both fins is cat off by the 

 edoe of the stone, so that its actual lens^th and number of 

 segments are not seen. Some small radials are seen attached 

 to the preaxial side of the first two segments, — none on the 

 others, or on the postaxial side of the stem. 



The interest of this specimen is extreme, as it is at least 

 capable of bearing the interpretation that we have here a 

 veritable uniserial archipterygium intermediate between the 

 truly biserial one of Xenacanthus and the pectoral fin of ordi- 

 nary sharks. If this interpretation is correct, then, along 

 with Xenacanthus, this specimen is a witness against the 

 lateral fold theory of the paired fins at present so popular 

 with anatomists and embryologists. Into that question I 

 shall enter on another occasion ; meanwhile so much is clear, 

 that if we have before us the pectoral fin of Cladodus — and I 

 do not doubt that we have — the affinity between it and 

 CMamydoselachus is not quite so close as Mr Garman main- 

 tains, seeing that in his fish the pectoral fin shows the ordi- 

 nary arrangement of basal pieces, though the metapterygium 

 has two segments. 



What, then, of the Ctenacanthus theory ? No spine is seen 

 in connection with the East Kilbride Cladodus; but, as the 

 body is absent, spines may have been borne by the fish when 

 complete. Again, in the Eskdale Ctenacanthus the form and 

 structure of the pectoral fin is not shown ; and though I 

 interpreted its one imperfect tooth as " cladodont," I am 

 willing to leave that an open question. It may be hyhodont, 

 and the hybodont form, with its vertically compressed base, 

 must not be confounded with the cladodont type with its 

 base horizontally flattened and irregularly elliptical or reni- 

 form. And in one of the instances which have been advanced 

 to prove the connection of Cladodus with Ctenacanthus, a 



