100 IOWA GEOLOGICAL SURVEY 



Now it has been shown by Dr. Bashford Dean that in Clad- 

 oselache the paired fins were mere balancers with a more ex- 

 tended base line than is usual. The series of parallel carti- 

 laginous rods, which in a ' primitive condition supported the 

 lateral fin-fold, exist practically unmodified in the pelvic fins, 

 simply clustered and partly fused within the body wall of the 

 pectoral fins. Dean, Cope, and others are of the opinion that 

 there is a tendency in the pectoral fin for the hinder end of the 

 row of basals to rotate outwards — a process which would reduce 

 the point of attachment of the fin to what was originally its 

 front angle. The outwardly turned row of basals would in this 

 case correspond with the median axis of the well known paddle 

 in Ceratodus and Pleuracanthus, and one may without difficulty 

 conceive of a fringe of cartilaginous rays developing quite sec- 

 ondarily along the hinder border of this axis. Hence, as argued 

 by Dean and Cope, the modern tribasal or dibasal shark's fin 

 cannot have evolved from the paddlelike "archipterygium", but 

 these two kinds of fin must have arisen independently from the 

 "ptychopterygium", as the arrangement has been appropriately 

 termed by Cope.* 



Complete skeletons of Cladoselache have been found only in 

 the Cleveland shale (Upper Devonian) and Waverly of Ohio. 

 Dismembered remains of the genus have been found also in 

 the Portage of western New York State, and numerous detached 

 teeth of the form common to both Cladodus and Cladoselache are 

 distributed throughout the Carboniferous rocks of all parts of 

 the world. Forerunners of Cladodus begin to appear as early 

 as the Middle Devonian ("Corniferous" of Ohio), and various 

 small, conical teeth that have been named Protodus, Doliodus, 

 etc., are known from the Lower Devonian of Campbellton, 

 New Brunswick. The precise position of these detached dental 

 structures cannot be fixed with certainty, since there is reason 

 to believe that several primitive genera, not necessarily of the 



*For interesting discussions of this subject, one may consult the following: Mol- 

 lier, S., Die paarigen Extremitaten der Wirbeltiere. I. Das Ichthyopterygium. 

 Anatom. Hefte, 3, 1893; Dean, B., Contributions to the Morphology of Cladosel- 

 ache. Jour. Morphol. 1894, 9, 87-114; A new Cladodont from the Ohio Waverly. 

 Trans. New York Acad. Sci. 1894, 13, 115-119; The Fin-fold Origin of the Paired 

 Limbs, in the Light of the Ptychoptervgia of Paktozoic Sharks. Anatom. Anzeig. 

 1896, 11, 673-79; Woodward, A. S., the Problem of the Primeval Sharks. Nat. 

 Sci. 1895, 6, 38-43; Ibid, 1892, 1, 28-35. 



