VASCULAR SYSTEM. 



253 



Tliis incomplete septvtm is continued into the ventricle. The conus pre- 

 sents traces of eight trans\-erse rows of valves. The number in each row- 

 varies in different parts of the conus (posteriorly about eight, anteriorlv 

 four or even one). The valves of the first two rows and those of one 

 longitudinal row are larger than the others. The longitvidinal row referred 

 to lies in the ventral middle line in the posterior part of the conus and lo 

 the riglit side in the anterior region (in consequence of the spiral twist 

 whicli the anterior part of the conus has imdergone). These valves are 

 so enlarged and united with one another by fibres that they form the 

 longitudinal valve above referred to. 



In the heart of Protopterns the arrangements are very similar, except 



Fig 131. .4. ventral B. — Median section through the contracted 



view of heart of heart of Ceratodus sliglitly diagrammatic 



Ceratodus (after (after Boas) ; the posterior part of the 



Boa.s). at, auri- conns only is shown, at, auricle ; co, 



sle ; ve, ventricle r conus ; ve, wall of ventricle ; svp, left 



c, conus; ]a to 4a, (pulmonary) division of sinus venosus ; 



arteries to the four si', larger right (systemic) division of 



branchial r.rches sinus ; iv, imperfect auricular septum, the 



springing close to- cavity of the auricle can be seen ui the 



gether from the section passing on the ventral side of the 



ventral aorta va. free edge of this ; 1^, P, the 5th and 6th 

 valves of the row constituting the longi- 

 tudinal fold 



that (1) the valves of the longitudinal row are more completely united to 

 form the longitudinal fold ; (2) there is in the anterior part a second 

 longitudinal fold formed from two valves of the two first transverse rows ; 

 and (3) the other smaller valves of the conus are much less numerous than 

 in Ceratodus. 



In Lepidosiren the two longitudinal valves of the conus completely 

 divide it into two parts, and the ventricular septum indicated in the other 

 two genera is complete except for a small pore between it and the conus 

 septum. The auricular septum is also well developed, though the meshes 

 in the ventricular end of it may admit of some iiitercommunication 

 through it. 



The ventral aorta gives off three arteries on each side. The 



