548 MULLER ON THE STRUCTURE OF THE GANOIDS, 
rounded by a muscular coat, their mucous membrane merely ex- 
hibits very delicate, parallel and minute folds in oblique rows. 
In Lepisosteus the muscular substance forms fleshy bundles 
lying in the furrows of the cellular divisions; but the arrange- 
ment of the small cells is quite independent of trabeculz carnez. 
All the Ganoids, like the Selachii, possess a thoracic gland. 
This is the gland which Stenonis first discovered in the Rays 
(Anat. Raie), and lies in the centre of the frame-work of the gills, 
between it and the gill-artery. It was recently described by 
Simon in the Sturgeons as the thymus gland, and also exists in 
the same situation in Polypterus and Lepisosteus ; it is usually 
single, but in Polypterus is double ; it agrees perfectly in micro- 
scopic structure with that of the thymus gland. 
The vascular glands on the surface of the heart in the Stur- 
geons also occur in the Spatularie. 
Agassiz, Valentin and Van der Hoeven, in their descriptions 
of the intestines of Lepisosteus, have not noticed the spiral valve 
of the alimentary canal. As the intestines had been removed 
from the specimens which I examined in the Zoological Museum ~ 
at Paris, and having been placed in the anatomical cabinet were 
not at hand, I was obliged to leave this point undecided. How- 
-ever, in examining the North American specimens, I find that the 
spiral valve is always present ; itis only rudimentary both as re- 
gards its length as well as its height. It is absent in the greater 
part of the canal, commiencing about the extremity just before 
the large intestine; it makes only three spiral convolutions and 
is very slightly elevated, so that functionally (increase of surface) 
it has no action, and is merely an indication of the general plan 
of organization of the Ganoids. We also thus see that the spiral 
valve in those fishes which are furnished with it, the Ganoids, 
Sirenoids and Plagiostomi, begins to be developed at the junc- 
tion of the chylopoietic canal with the large intestine, and that 
it increases in length from below upwards, not from above down- 
wards. It attains its maximum of development when, as in the 
Plagiostomi and in Polypterus, it extends as far up as the spot 
where the bile is poured out, or as far as the duodenal portion 
ot the intestine. 
The existence of the spiral valve belongs henceforth to the 
absolute or general characters of all Ganoids, but nothing of the 
kind is observed in any of the osseous fishes. Numerous rows 
of valves in the arterial trunk, a muscular layer on the same, and 
