The Galena and Maqnoketa Series. — Sardesoi). 183 
Meek, Pal. Ohio, vol. 1, p. 72, pi. 5. tig. 8,f-h, ( 1873, ) which occurs 
at 150 feet above low water line of the Ohio river at Covington, 
Kentucky, and this one is there the predecessor, possibly the 
ancestral form of P. recedens (Sar). It would, however, very 
much raise our confidence in such a theory of derivation, if, 
in any one case, these species or varieties and forms, as they 
have been called, had been traced by intermediate steps from 
one to the other. 
The evolutionary changes took place in one of two ways, 
either quickly in a few obscure individuals, or gradually in 
perhaps many. In the latter case it is necessary to assume 
either that great time intervals are represented between cer- 
tain beds, or that the evolution took place in other regions, the 
"varieties" 7iiigrating sucessively into this area. If the evo- 
lutionary changes were sudden, the several species are certain- 
ly "distinct," but if gradually'then. too. they are distinct so 
far as is yet known. It is highly improbable that the differ- 
ences between these species are due to evolution in this area 
during time intervals, i. e., no such time intervals can be sup- 
posed in accord with the known facts. 
It would be easy to confuse one of the above species with 
another if not kept separate by the collector, because the ex- 
ternal characters are combined with many extreme differences 
due to accident of growth, and because the interiors are not 
only very liable to changes throljgh maceration, but also the 
differences between a young mature shell and a senile one 
of the same species are more striking than the specific differ- 
ences, between two of different species. 
One or two distinguishing features ma}^ be given here. 
Plectarnbonites .sericea and P. minnesofensis have the stria? 
less divergent from the margins of the cardinal areas, and 
the process between the two muscle scars on each half of the 
dorsal valve is relatively very small even in senile shells, as 
compared to the Maquoketa species. P. sericea is larger and 
correspondingly more coarsely plicated and much less convex 
on an average than P. minnesotensis is. Of the others, P. 
recedens is the synthetic type, is moderately convex and is 
about as large as P. sericea although more convex. P. saxen 
is large, very moderately convex and thin, P. pr<fcosis is xery 
convex, with heavy hinge area, and is small, /'. aspcra of Cin- 
