496 PROF. EAT LA^fKESTEE ON THE HEAETS OF 



therefore spiral) valve, be at once examined. It will be seen from that diagram that 

 the arterial cone may be represented as a cylindrical tube which is spirally twisted on 

 itself leiotropically — that is, in such way that in ascending the spire one would turn to 

 the left. There is a lower vertical limb, and an upper vertical limb, and a transverse 

 oblique segment connecting the two vertical limbs. A longitudinal muscular fold 

 projects from the interior of the tube, and makes with it a spiral leiotropic turn. This 

 fold is the single " longitudinal " or " spiral valve " of the arterial cone of Ceratodus. 

 It is not continued in that animal into the upper limb of the cone, but stops short in 

 the transverse segment (fig. 7c). In fig. 2 the transverse portion of the arterial cone is 

 partly exposed (though much of this cavity lies deeply behind what is there exposed), 

 and the upper termination of the spiral valve {Sp. v) is seen. If we could follow the 

 cavity, we should pass in the direction indicated by the dark shading behind this bit 

 of the spiral valve, and then plunging downwards we should emerge in the cavity of 

 the ventricle at the point indicated by the deep shadow where the dotted line termi- 

 nates inferiorly. This dark shadow, then, indicates the lower end of the arterial cone. 

 Its deep-lying wall is seen to form a broad arch-like flap, which is tough, though 

 membranous {AW). Immediately below this flap a second dark shadow indicates 

 another recess, which is, in fact, the passage from ventricle to auricle. This arch-like flap, 

 which is the inferior internal wall of the arterial cone, may therefore be regarded as 

 the auriculo-ventricular valve. It has been fully recognized by Dr. Giinther ; and an 

 exactly similar arch-like valve is described by Owen in Protopterus and by Hyrtl in 

 Lepidosiren (see figs. 10 & II). The cut flap of the ventricular wall, which is reflected 

 to the right, shows chiefly cut muscular fibre on its surface ; but the more central part 

 is the natural right wall of the ventricular cavity, and is marked by a meshwork of 

 trabeculae camese. The floor of the ventricular cavity, as exposed in the dissection 

 (fig. 2), is seen to present a smooth convex prominence {FC), which, taking its origin in 

 the muscular substance at the apex of the ventricular cavity, passes as a solid ridge 

 beneath the auriculo-ventricular archway. Its actual form and extension are best seen 

 in fig. 4. This remarkable structure has been recognized by Hyrtl in the heart of 

 Lepidosiren, and by Giinther in Ceratodus and Protopiterus. 



It is no doubt efficient in causing a closure of the auriculo-ventricular passage, being 

 brought up against the arch-like flap or curtain during the ventricular systole. Of its 

 extension into the auricle I shall speak in describing the further dissection exhibited 

 in fig. 4. 



The dotted line running in figure 2 from the front edge of the spiral valve across the 

 floor of the transverse part of the arterial cone, and then across the cut muscular sur- 

 face of the cone to its lower opening, indicates the next step in the dissection. A cut 

 being made as indicated by that line, we obtain the appearance drawn in fig. 3. 



In fig. 3 the lower limb of the arterial cone is opened, and the transverse portion (at 

 the left-hand side of the figure) is more widely exposed than in fig. 2. The lower 



