868 THE PALEONTOLOGY OF MINNESOTA. 
[Protowarthia rectangularis. 
at least some of the forms of Protowarthia is delicately cancelled is not now consid- 
ered of special consequence. A more significant fact is that while Protowarthia is 
almost or entirely restricted to the earlier periods of the Paleozoic, Warthia, if the 
geological record is even approximately reliable and complete, did not make its 
appearance till just before the close of that age. Here we have a difference in 
geological distribution that can be accounted for in only two ways: either the 
Permo-Carboniferous Warthia is distinct from Protowarthia and originated in some 
nearly contemporaneous (say Carboniferous) type, or we must assume that the two 
are the same and that the connecting species, which in that case must have existed 
during the Upper Silurian, Devonian, Lower Carboniferous and Coal Measures, were 
either destroyed or remain yet to be discovered. Considering the abundance of 
other bellerophontids in these strata, the latter alternative is certainly very 
improbable. 
On page 856 we express the opinion that Warthia (also Euphemus and Mogulia) 
is a result of the decline and approaching extinction of the Bellerophontide. As 
stated there, such a return to primitive characters is quite in accordance with 
theories that are rapidly entering the realm of facts. We would have been pleased 
to discuss the questions involved at greater length, but the lack of space necessitates 
a postponement to some perhaps more appropriate future opportunity. 
Compared with other bellerophontid genera Protowarthia is distinguished by 
the absence of a slit-band and the width and size of the sinus in the outer lip; 
Bellerophon has a distinct slit-band and centrally angular sinus and slit; in Owenella 
there is an umbilicus, the sinus is much shallower and centrally subangular, and 
the form of the shell more nearly globose; Bucanopsis has a slit-band, and so has 
Bucania, coupled with a large umbilicus; finally, in Oxydiseus the whorls are 
laterally compressed and sharply keeled and the shell broadly umbilicated. 
Respecting the species of Protowarthia, they are among the most characteristic 
and abundant fossils of the Lower Silurian system. None are known in the Upper 
Silurian and only a single Devonian species. The last, moreover, is doubtfully 
referred to the genus. 
PROTOWARTHIA RECTANGULARIS, %. Sp. 
PLATE LXIII, FIGS, 15—20. 
Shell of medium size, rather closely coiled, leaving only a small umbilicus in 
the cast which, in the shell itself, is probably closed as in P. cancellata; outer volu- 
tion sharply rounded dorsally, with a nearly flat or gently convex slope on each 
side, then turning rather abruptly into the umbilicus; aperture transverse, subtri- 
angular, about twice as wide as high, the width but little less than the hight of the 
