On the Organization of Coenogonium. 427 
The crown of the teeth in all three genera is thickly coated 
with enamel; and they have all a ceneral family resemblance. 
They appear, however, to be much more nearly related to the 
Petalodonts than to Or odus, as suggested by Prof. Agassiz. 
The convex surface of the crown in fr ont, and the corresponding 
concave surface behind, form a petal-shaped tooth very similar 
to that of some of the Petalodonts ; they are distinguished by 
the deeply denticulated or pectinated cutting-surface. Whilst 
agreeing in the possession of a family likeness one with the 
other, they exhibit considerable generic distinctions. The 
genus Harpacodus differs from the other two in its thick, 
strong crown and the almost straight contour formed by the 
enamelled tips of its denticles. ‘The constricted, tumid, and 
prominent bony base is quite different from the flattened bases 
of Otenopetalus and Ctenoptychius. ‘The denticulation of the 
three genera is sufficiently distinct. Ctenoptychius, though 
possessing about an equal number of denticles with Harpa- 
codus, is distinguished by their peculiar irregularly acuminate 
arrangement, whilst in Harpacodus they extend almost 
straight across the tooth. ‘The denticulation of Ctenopetalus 
is easily discriminated by the large number of denticles and 
their comparative smallness in proportion to the size of the 
tooth. 
XLUL— The Organization of Coeenogonium, and the Theory of 
Lichens. By Dr. J. MULLER *. 
THE genus Cenogonium, established in the class of Lichens 
by Ehrenberg in 1820, at present consists of about twenty 
species, all of which grow in the warmer regions of the two 
hemispheres. Their fruits, or apothecia, and their spores are 
like those of the section Biatorina of the genus Patellaria, 
while their thallus, or vegetative part, has a "totally different 
structure, approximating the genus to Graphis. 
The constituent elements of this thallus are filiform, not 
much branched, more or less parallel to one another, and 
very loosely combined into a felted mass, which, according to 
the species, may have the form of a small sod or flattened 
cushion, or may become developed horizontally in the form 
of a fan about 2-8 centim. in diameter; and on examining 
them one is struck with the great resemblance which these 
filamentous elements show to the filaments of the Conferve. 
Tubes (filaments) about 5-30 w in width, according to the 
* Translated by W. 8. Dallas, F.L.S., from the Bibliotheque Univer- 
selle, Arch. des Sci. 15th October, 1881, p. 370. 
30* 
