34-t AXXALS XEW YORK ACADEMY OF SCIEXCE8 



Eor were the basal cartilages widelv protruded. Consequently the paired 

 fins were used rather as steering planes, which could be raised or lowered 

 or gently undulated, rather than as paddles, capable of twisting about a 

 narrow base. 



The typical elasmobranchs have followed a conservative line in the 

 evolution of their fins : in the distal portions of all the fins they have 

 developed horny dermal rays or ceratotrichia, which lack a bony or cal- 

 careous basal portion, but serve well as the flexible elastic fin-web. In 

 the typical sharks the basal cartilages have become more or less widely 

 protruded from the body-wall, the base has shortened and the posterior 

 border of the fin has become sharply exserted or entirely freed from the 

 body-wall, with more or less rearrangement of the basals and radials: 

 consequently the pectoral fins of sharks and still more of cliimaeroids have 

 become very efficient paddles, capable of a wide range of movements. In 

 the skates, on the other hand, after the primary shortening of the base 

 and coalescence of some of the basal rods, there was a secondary antero- 

 posterior widening of the fin. a multiplication of the radials, with dichot- 

 omization of the distal ends, and a great forward and backward exten- 

 sion, with corresponding emphasis of the power to undulate the outer 

 border. The pectoral girdle accordingly becomes a stout depressed hoop 

 and secures a firm articulation dorsally with the vertebral column. All 

 such improvements in the median and paired fins have been accompanied 

 bv a g'reat strensrthenino- of the axial skeleton, and bv the functional 

 replacement of the notochord by the calcified centra, developed in and 

 around the perichordal sheaths. 



The pleuracanth sharks are of historical importance in any discussion 

 of the early history of the paired limbs. Their pectoral limbs, recalling 

 the "archipterygial"' type of Gegenbaur, were assumed to be the most 

 primitive form known and the vague resemblance of the pectoral girdle 

 and fin to a gill-arch and its extrabranchial rays were by no means over- 

 looked. But it is now coming to be realized that the pleuracanths, which 

 are of Permocarboniferous age, were highly specialized, aberrant sharks, 

 living in fresh-water, along with branchosaurs, microsaurs and other 

 swamp -dwelling types, wriggling about with their long Gyninotus-like 

 body or paddling with their Ceratodus-like pectorals. Whether the 

 diphycercal tail of pleuracanths is a primitive structure is doubtful. 

 The loss of the primitive heterocercal tail and the assumption of the 

 gephyrocercal form in Ceratodus, Protopteriis, Lepidosiren and certain 

 swamp-living teleosts (s^Tubranchoids, g^Tiinotids, g}Tnnarchids, etc.) is 

 an indication not of primitiveness, but, as DoUo has -shown, of degen- 

 erative specialization. On the other hand the tail-fin of pleuracanths 



