500 PEOF. EAY LANKESTEK ON THE HEARTS OF 



ventricular curtain into the auricle) ; but instead of ending abruptly in the auricle, it 

 is continued across the sinu-auricular floor in the form of a strong fibrous ridge, and 

 can actually be traced into the tough fibrous walls of the large vena cava, which, sur- 

 rounded by liver, opens mesially into the hinder part of the sinu-auricular cavity. 



It would thus seem that the basal fibro-cartilage, " eine Vorrichtung, die noch bei 

 keinem Wirbelthiere beobachtet wiirde," as Hyrtl remarked when first describing it in 

 Lepidosiren, has its most extensive development in Ceratodus (it is shown throughout 

 its course in fig. 4), and its smallest in Protoptenis ; whilst in Lepidosiren it acquires a 

 special relation to muscular trabeculse of the auricle, which gives it a share in the 

 formation of the imperfect auricular septum present in that alone of the three genera 

 of Dipnoi. 



II. Heaet of Protopterus (Lepidosiren) annectens. 



Professor Owen, in the description of the anatomy of this form, has not given special 

 illustrations of the general appearance and interior structure of the heart ; and, I believe, 

 it has not been figured by any later writer. In a monograph which contained so much 

 that was new and of the greatest importance, it is not surprising that the smaller 

 details of one organ should be passed over. The main features of the structure of the 

 heart (excepting the basilar fibro-cartilage) were briefly described by Owen ; his atten- 

 tion was especially given to the structure of the conns arteriosus, concerning which he 

 notes the absence of watch-pocket valves, the spirally-twisted shape, and the presence 

 of longitudinal folds or valves. 



In figs. 8, 9, 10, 11, 12 I have given drawings of the entire heart oi Protopterv,s^ 

 and of dissections. 



In figui-e 8 a front view of the heart removed from the pericardium is given. It is 

 more symmetrical than that of Ceratodus, and difi'ers most markedly from the latter in 

 the presence of large auricular appendages, and in the constriction of the neck of the 

 arterial cone. A fibrous band connects the apex of the ventricle with the pericardium, 

 as in Ceratodus. 



On turning the same heart we obtain a more complete view of auricle and sinus (fig. 9). 

 The sinus is elongated posteriorly, and has tough fibrous walls where it becomes con- 

 tinuous witli the large median vena cava. Towards the auricle its walls become much 

 more delicate, and are loosely folded in the collapsed condition. Externally there is 

 no marked constriction between the sinus and the median portion of the auricle. The 

 lateral expansions of the auricle are very voluminous, and are vesicular or even digi- 

 tate. They extend around the ventricle, and fit with their upper processes into the 



' I must poiut out here that the observations recorded in the present paper on the hearts of Ceratodus, 

 Protopterus, and ChimcBra have, owing to special circumstances, been made on excised hearts. I have not, 

 therefore, in any ease entered into the question of the number and position of the openings of vessels into the 

 auricular sinus, nor of those from the conus. 



