xii rilVLUM CIIORDATA 



jointed rods of cartilage on 

 either side of it. This form 

 of fin-skeleton, which occurs 

 in certain groups of fossil fishes 

 as well as in Dipnoi, has been 

 termed the archipterygium. 

 The notochord is persistent 

 and the cranium (Fig. 229) 

 consists of a mass of cartilage 

 with little ossification, but with 

 the addition of a number of 

 membrane bones ; the skull is 

 autostylic, the lower jaw arti- 

 culating with a palato-quadrate 

 process (pal,\ corresponding 

 to the palato-quadrate of the 

 Dog-fish, but immovably fixed 

 to the side of the skull. There 

 are four or five cartilaginous 

 branchial arches (br.\ The 

 gills are covered over by 

 an operculum. A cloaca is 

 present, and the intestine con- 

 tains a spiral valve. The 

 structure of the heart is more 

 complicated than in ordinary 

 fishes, owing to the sinus ven- 

 osus and the auricle being both 

 imperfectly divided into two 

 parts. There is a contractile 

 conus arteriosus, which has 

 a spirally twisted form, and is 

 partly or completely divided 



D D 2 



