VASCULAR SYSTEM. 



253 



Tills incomplete septum is continued into the ventricle. The conus pre- 

 sents traces of eight transverse rows of valves. The number in each row 

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

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

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

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

 the right side in the anterior region (ui consequence of the spiral twist 

 which the anterior part of the conus has undergone). 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 Protopterus the arrangements are very similar, except 



iifp 



FlO 131. A. ventral 

 view of heart of 

 Ceratodus (after 

 Boas), at, auri- 

 cle ; ve, ventricle ; 

 e, conus; la to 4a, 

 arteries to the four 

 branchial arches 

 springing close to- 

 gether from the 

 ventral aorta va. 



B 



B. — Medi.in section through the contracted 

 heart of Ceratodus slightly diagrammatic 

 (after Boas) ; the posterior part of the 

 conus only is shown, at, auricle ; co, 

 conus ; ve, wall of ventricle ; svp, left 

 (pulmonary) division of sinus venosus ; 

 sv, larger" right (systennic) division of 

 sinus ; w, imperfect auricular septum, the 

 cavity of the auricle can be seen in the 

 section passing on the ventral side of the 

 free edge ol this ; P, 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 Lej idosiren the two longitudinal valves of the coniis completely 

 divide it into two parts, and the ventricular septimi 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 intercommtuiication 

 through it. 



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



