On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 451 
Genus—Helodus, Agassiz. 
“Sous cette denomination je comprends toutes les dents de Psammodus, dont la 
surface est parfaitement lisse et le centre plus ou moins renflé en forme de céne 
obtus. Ces dents sont tantot allongées et arrondies avec un seul renflement au 
milieu, tantét elles présentent une série de cénes obtus, dont celui du milieu est 
plus élevé, tandis que ceux des cdtés vont en diminuant de grandeur, tantét en fin 
un simple céne plus ou moins saillant. Toutes les espéces connus ont été trouvées 
dans les terrains houillers.”’—(Agassiz, “ Poiss. Foss.,” t. iii., p. 104.) 
Prof. Agassiz originally figured the species of the genus Helodus as belonging 
to the genus Psammodus. The latter embraced al] those teeth whose crown 
was composed of minute vertical tubes, and whose general structure bore a close 
relationship to that of the Cestraciontes. The surface of the tooth was more or 
less smooth, but punctate by the open ends of the pores of the crown, without 
corrugations or striations, unreticulated and devoid of either crests or longitudinal 
and transverse prominences. The discovery of an increased number of specimens 
and the great variety of teeth included in the above definition led Prof. Agassiz to 
divide the genus Psammodus into several others. They were as follows :— 
I. Helodus. 
II. Chomatodus. 
III. Psammodus. 
IV. Cochliodus. 
V. Strophodus. 
During the Geological Survey of Illinois, a number of new species of Helodi 
were discovered, and amongst them some specimens which exhibited a close 
relationship between teeth of Helodus and Cochliodus. The specimens are 
described in the ‘“ Paleeontology of Illinois,” Vol. I., pp. 88-91, and consisted of 
the upper and lower teeth of a large species of Cochliodus and a number of teeth 
of Helodus ; in addition, the jaws are also preserved to some extent. The dis- 
covery of any semblance of jaws with the teeth attached is an occurrence of such 
importance that I venture, though rather lengthy, to give Mr. Worthen’s descrip- 
tion in his own words. “ Fragments only of the jaws are visible on the specimens 
contained in the collection and such as are quite insufficient for determining their 
form : they are now thin and flattened and much distorted, showing they had little 
firmness or rigidity, and were, doubtless, for the most part, cartilaginous, though 
it is possible in part ossified. They do not show a true bony structure, but 
exhibit on fracture a fine granular composition, such as we have before seen 
accompanying the more distinctly bony portions of the remains of cartilaginous 
fishes, indicating, perhaps, a cartilage through which were disseminated innumerable 
granules of ossific matter. 49 
“The group of teeth impacted together includes at least four distinct and 
different forms, of which the surface markings, microscopic structure, colour, &c., 
are precisely the same throughout. These are, Ist. Large, strongly enrolled teeth, 
