NILS ROSEN, STUDIES ON THE PLECTOGNATHS. 3 



the Plectognaths, except about Orthagoriscus and a few words 

 on Balistes. In Spheroides the sinus venosus is notched be- 

 bind giving almost the appearance of two large vessels united 

 just at their entrance into the auricle (Fig. 1. sv.) The 

 auricle {atrium) is of a very irregular sbape (a). I think 

 it should best be compared with a somewhat irregular 

 pyramid turning its concave base., in which the ventricle 

 is situated, towards the right, the top towards the left. 



.t.f>^- 



vis 



b- 



£L 



C d ii. 



Fig. I. Heart of Spheroides (Tetrodon) testudineus L. a, ventral view ; 

 h. from the right side ; c, from the left side ; d, transverse seotion just be- 

 fore the auriculo-ventricular opening. e, auricle, showing aur.-ventr. opening. 



1, 2, 3. arteria branchialis af ferens prima, secunda, tertia ; a, auricle 

 (atrium); sv, sinus venosus; v ventricle; avo auriculo-ventricular opening; 

 vcad, vena card. ant. dext. ; vh vena hepatica; vfi vana jugularis inf. ; vssid) 

 vena subclavia sin. (dext.); vis (d) vena lateralis sin. (dext.); vsp vena sper- 

 mat ica. 



At about the centre of the base the auriculo-ventricular 

 opening will t hen be seen. When seen from the ven- 

 tral side the auricle is triangulär. Near the left angle there 

 is a feeble furrow for the left lateral vein. The ventricle (v) 

 is cylindrical, its caudal portion being partly enclosed by the 



