No. 432.] STUDIES OF GASTROPODA. 939 
Trochonema vagrans Ulrich and Scofield (Fig. 15), from the 
Stones River group, represents a phylogerontic type among 
the carinated naticoid gastropods, while Dyeria costata (James), 
from the Upper Lorraine of Cincinnati, 
represents a phylogerontic type of the 
group of simple naticoid shells with 
depressed spire. In the Silurian, P/atyo- 
stoma (Diaphorostoma) niagarensis fre- 
quently shows individual gerontism by a 
loose outer lip. Platyceras miagarense 
represents a phylogerontic type, possibly 
derived from the former. In the Devonian 
the phylogerontic non-coiling Platycera 
abound. We find all degrees of coiling, 
from the close-coiled non-umbilicate Dia- "9 '* RA gore 
phorostoma, which appears with slight showin Paley of Mi 
vol. Fig. 
modifications throughout most of the  xesota, pl. Ixxii, 
26.) 

series, to the straight * Orthonychia,” 
which appears as a terminal member in most groups. Very 
often a number of species of “ Platyceras” can be traced to a 
species of Diaphorostoma or Strophostylus occurring with them, 
the gradations being perfect. From such evidence it appears 
that the numerous species classed together as Platyceras must 
be split up into groups, each of which 
has been derived from a close-coiled 
ancestral species, probably within the 
same geological horizon. If so, the 
name Platyceras becomes meaningless 
for generic purposes. The great diffi- 
culty which besets the proper breaking 
up of what appears most certainly to be 
PN Sec Twice eeut A OOD of polyphyletic origin lies in the 
U. and S., illustrating geron 
final whorl. (After Ulrich, 7. small number of ornamental characters 
læontology of Minnesota, vol. which can be made use of in tracing out 
ii, pl. ioiii. Fig. 13.) i 
relationships. In many Platycera spines 
appear, but these cannot be regarded as clues to affinity, since 
spinous types may and do arise in any group. What seems to 
be the right method of procedure was made use of in the 

