PAIRED FINS 109 



weighty one, is that it becomes difficult to avoid the supposition 

 that the archipterygial type of fin has been independently 

 developed in the Pleuracanthodii and the Dipnoi (p. 244). 



The development of an outstanding basal stem, or axis, bearing 

 peripheral radials, is clearly seen in the median fins of certain fossil 

 and living fish (A. Smith Woodward [503], Thacher [434], Mivart 

 [300], and Figs. 251, A ; 147). The dorsal fins of the Eajidae (Fig. 

 49) and the anal fins of the Pleuracanthodii are good examples. 

 The orthostichous arrangement of the radials in the paired fins of 

 the higher Teleostomes (p. 302 and Fig. 243) is almost certainly 

 secondary, and due to the reduction of the axis. 



It is important to notice, as a general primitive character, that 

 the endoskeleton of the fins typically has no articulations ; that is 

 to say, it moves as a whole, being articulated to the girdle, but is 

 not subdivided into regions moving on each other, as is the case 

 with the limbs of the Tetrapoda. Further, we find that, in the 

 more specialised fish, the endoskeleton has a tendency to become 

 reduced, and the exoskeletal dermal rays, on the contrary, become 

 increasingly important as supporters of the web of the fins. These 

 dermal fin-rays, or dermotrichia [175], are found in both the paired 

 and the unpaired fins of all Pisces. They form a very characteristic 

 innovation diagnostic of the Class, and will be described below 

 (pp. 122 and 212). 



Turning to the vascular system, we find a symmetrical heart lying 

 in a pericardium, situated ventrally, between the pectoral girdle and 

 the basibranchial plate underlying the oesophagus (Fig. 303). The 

 pericardial coelom is almost (Chonclrichthyes, Chondrostei) or quite 

 (Teleostei, Lepidosteiclae, Amiidae) closed off from the abdominal 

 coelom by a transverse septum. Here the right and left ductus 

 Cuvieri and the hepatic veins join to enter the sinus venosus. This 

 leads into a thin-walled atrium, opening into a muscular thick- 

 walled ventricle. There are two sinu- atrial and two atrio- 

 ventricular valves. The ventricle is prolonged forwards into a 

 contractile conns arteriosus, leading to the ventral aorta (Stohr 

 [42G], Boas [38-39], Rose [373]). Inside the conus are rows of 

 semi-lunar valves (Fig. 69). In the more specialised Teleostei the 

 base of the aorta is swollen into a non-contractile bulbus, and the 

 conus is reduced to a narrow strip supporting only one row of 

 valves (p. 363). 



As a primitive type of circulation we may take that of a 

 Selachian (Hyrtl, Balfour [27], Hochstetter [214-15], Dohrn 

 [114], Rabl [336], Parker [314-15]). The median ventral aorta, 

 morphologically a forward prolongation of that longitudinal ventral 

 ' subintestinal ' vessel of which the heart itself is a specialised 

 portion (p. 26), and like that vessel developing from originally paired 



