COCnr.IODONTID.^E. 
171 
teeth — which are nearly always, if not always, placed posteriorly 
upon the jaws. For convenience of description, the margin to which 
additions are made during growth will be termed the “ inner ” 
border, the opposite in-roUed margin the “ outer ” border ; the 
other pair of sides will be named the “ antero-latcral ” and “ postero- 
lateral ” respectively, and a line joining these will be transverse. 
The most generalized members of the family, Helodm, Pleuroplax, 
and Pseplunhis, will be treated first, the most specisilized genus, 
Beltoptyclnus, and some uncertain forms, being jilaced last. Pleuro- 
plax is the only genus in Europe certainly ranging throughout the 
Carboniferous Period ; and in the North- American beds Psephodus 
is stated not only to occur in the earliest, but also to have the 
greatest vertical range in the Lower Carboniferous 
Genus HELODUS, Agassiz. 
[Poiss. Foss. vol. iii. 1838, p. 104.] 
As originally defined by Agassiz, the generic name Ilelodus is 
applicable to all the detached anterior teeth of Cochliodonts {see 
p. 218) ; but, in the absence of definite evidence, it seems advisable 
to restrict the term provisionally to the type species, which repre- 
sents a genus still awaiting elucidation. This fish is ob\4ously very 
closely related to Pleuroplax, as shown both by the dentition and 
the dorsal fin-spine “ ; but in the known examples of the last-named 
genus all the teeth are described as fused into plates, while in the 
typical Helodm no such arrangement has been discovered^. If, 
indeed, the latter observation be confirmed, the genus does not 
strictly fall under the accepted definition of the Cochliodontidse, and 
some modification will bo required ; but there can be no doubt that 
in whatever family Pleuroplax bo placed, the type species of Helodm 
must follow. The dental crown is tumid, and the coronal surface 
coarsely punctate. 
Helodus simplex, Agassiz. 
1838. Helodm simplex, L. Agassiz, Poiss. Foss. vol. iii. p. 104, pi. xix. 
figs. 8-10. 
1861. Helodus simplex, J. W. Salter, Foss. S. Welsh Coal Field (Mem. 
Geol. Surv.— Iron Ores Gt. Brit. pt. iii.), p. 225, pi. i. %. 17. 
’ St. John and Worthen, Palaiont. Illinois, vol. vii. (1883), p. 59. 
’ J. Ward, [Proc.] N. Staffs. Nat. Field-Club, 1875, p. 221, 
’ E. H. Traquair, Geol. Mag. [3] vol. r. 1888, p. 84. 
