186 27/e American Geolociist. March, i«i.7 
two or one, or six to eight instead of three obtain. The sur- 
face is nearly smooth. The associated II. ainsliei N. H. W. 
in the Stietoporella bed (3) is broader, finer plicated, and 
averages larger than Ji. mi.nnesotensis. It has five to eight 
plications falling in the median sinus, but on the average 
there are six while li. minnesofeust's has oftenest thr^^e or four. 
The two species are really entirely distinct, although no one 
known feature can be taken as an unfailing distinction. 
In the Fucoid bed (5) are found a few Rhynchonella plerm 
Hall — Chazy formation, of New York — and they are larger, 
proportionately longer, with less sharply defined median fold 
and sinus than the largest It. niinnei^ofeiisis^ but both have 
smooth surfaces and might if mixed together be wrongly re- 
ferred to one variable species. Likewise RIni iirltniiella incre- 
bescenx^. sensu stricto, which is found in beds Nos. 5 to 7, 
and taking the place of It. riu'inie.sof.eusi.s, not only can be but 
has been persistantly confused with the last named species. 
They are nearly alike in size, but JL increbesceiis has the 
shell smooth only up to a certain adult stage, then concentric, 
projecting, laminar structures or growth lines developed like 
those familiarly known on Atrypa reticularis L. although in 
this case never forming hollow spines on the plications. The 
median sinus embraces three, rarely two or four plications 
and of these the median one is not rarely largest. Measuring 
along the median line, the dorsal valve of 7?. incre.hesceuti is 
longer proportionately than that of B. rninne.'iotensis. Ac- 
quaintance with both species makes their separation easy. 
Jihynchonella var. laticosfatn W. &. S. which is found in the 
Orthisiiia bed (6) associated with li. /«cre&e*ce».v, is not found 
in all zones with the latter, but in f>ne only, in the Galena 
series although its distribution is wide, being evidently rep- 
resented in the Trenton of Kentucky and of New York. Its 
plications are strikingly broad, and large, and its surface is 
always smooth apparently because at the adult stage growth 
is arrested and senile walling up or dorso-ventral growth of 
shell at the anterior margin follows where in M. increbescens 
the fimbriated portion of shell growth begins. Winchell and 
Schuchert describe it as a variety understanding it to be not 
distinctly separable evidently and in this they are right, but 
whether it is an oft repeated sport, or a true variety, or a 
