340 AN INTRODUCTION TO THE STUDY OF FOSSILS 



extending from the gills to the tail, giving rise to the paired fins, 

 — the pectorals and the ventrals or pelvic fins (Fig. 144) . These 

 fin folds, probably developed as balancing organs, were supported 

 by rods of cartilage extending outward from the vertebral arches. 

 Gradually the fins became shortened, the cartilaginous rods 



mo. ^UUUts 



df.r. 



d.f.% 



Fig. 144. — Ideal diagram representing the probable development of the fins. A, 

 fish with a continuous fin-fold. B, fin-fold broken up into distinct fins, o./., anal 

 fin; an., anus, in position midway between the two pelvic fins; c./., caudal fin; 

 d.j. I., anterior, and d.j. 2, posterior dorsal fins; ear, location of the auditory organ, 

 — the semicircular canals (functioning both in hearing and in the maintenance of 

 equilibrium) ; mo., mouth; na., nasal opening; pec.f., pectoral fin ; pel.f., pelvic 

 or ventral fin ; pin., pineal eye ; a.f., d.f. i, d.f. 2, and c.J. are unpaired fins, while 

 pec.f. and pel.f. are paired fins. 



supporting them became crowded together though still parallel 

 (as in the Pleuropterygii of the early Paleozoic) ; next followed 

 either a fusion of the basal supports of each fin into one (as in 

 the Ichthyotomi of the late Paleozoic, in the Paleozoic and Meso- 

 zoic Crossopterygii and in the living Dipneusti) or the carti- 

 lages became rudimentary (as in the Acanthodii of the Paleo- 

 zoic and the Actinopterygii of Paleozoic time to the present). 



Caudal fin. — When the tail fin is divided into two somewhat 

 equal lobes and the vertebral column extends nearly to its 

 end, it is most primitive and is called diphy cereal (Fig. 147). 

 When the vertebral column extends into the upper lobe, making 

 it much longer than the lower one, the caudal fin is called hekro- 



