8 ARKIV FÖR ZOOLOGI. BAND 7. NIO 25. 



question without knowing anything about the development, 

 but it seems me more probable that this character is a pri- 

 mitive one that has been preserved in Orthagoriscus, while 

 löst in the other Plectognaths and perhaps in all other Tele- 

 osts, than a character independently' developed within the 

 Orthagoriscus-gr ou-p. The greater number of valves in the 

 conus arteriosus is much more easy to explain. It is not 

 exceptional among the Teleosts, both in Xiphius^ and Salmo^ 

 the same number is found. It has evidently been shown by 

 several authors, that the conus arteriosus primitively was 

 an organ of a large size, provided with many valves arranged 

 in several rows each containing a varying number at least 

 more than two. In the evolution towards the Teleostean 

 stock this organ has become highly reduced, being represented 

 in this order only by a vestigial part between the ventricle 

 and the bulbus arteriosus. Parallelly with this reduction the 

 number of valves has decreased. In all Teleosts, except A Ibula, 

 there is only one row persisting, and the number of valves 

 in each row is only two, except in the few^ forms named 

 above. From this I think, we have all reason to consider 

 the existence of four valves in Orthagoriscus as an ancient 

 character. The other fact concerning the heart in the Plec- 

 tognaths that is of interest is the relative position of the 

 auricle and ventricle in Spheroides. In all the descriptions of 

 the heart in Teleosts, which I have seen, the auricle is always 

 stated to be placed dorsally to the ventricle, and the same 

 seems to be the condition in the Ganoids and also in the 

 Selachians. In Petromyzon, however, we meet with an arrange- 

 ment of the auricle and the ventricle just like that I have 

 described above in the case of Spheroides. In the young 

 stages of Petromyzon the heart-tube is straight, the auricle 

 situated behind the ventricle. This relative position changes 

 låter on, the auricle being turned to the left, the ventricle 

 a little to the right, thus both are placed laterally to each 

 other. This torsion is caused by the peculiar development 

 of the Cardinal veins in this form. In the Gnathostomes we 

 have a similar torsion but resulting in the auricle being placed 

 dorsally to the ventricle. Whether the arrangement in Sphe- 



^ According to Joh. MtfLLER. 

 - According to Boas. 



