1890.] The Naticoid Genus Strophostylus. 1117 
series, and is connected by a complete gradation of forms with 
the earlier and less specialized one. Later in the history of the 
most primitive form now known an exceedingly variable series 
(C) is given off, which assumes in the several species diverse char- 
acters. Some vary more towards the S. andrewsi type, while 
others tend towards the S. »iagarensis section. In the variable 
forms of S. Zurbinatus some significant phases are presented 
which suggest the relationship of these shells to certain other 
genera. In the extreme form appears an elevation of the spire 
that is unknown elsewhere in the group. Some examples show 
scarcely any thickening ofthe inner lip or columella, while others 
have these features well developed. 
It must be borne in mind that the scheme as here represented 
is intended to indicate merely the lines along which the several 
developments took place, rather than the phylogenetic history of 
the group. The correct determinations of the phylogeny of 
animals from paleontological evidence is attended with many 
difficulties. For, as repeatedly shown by Darwin and others, 
new variations tend to be transferred backward in the ontogenetic 
history of a species, and may disposess older characters. This, 
taken in connection with the fact that variant changes may occur 
in one part of an organism without materially affecting other 
parts, calls for extreme conservatism in passing judgment on phy- 
logenetic problems from evidence afforded by fossils. 
EXPLANATION OF PLATE XXXIII. 
Fic. 1. Strophostylus ventricosus Conrad, Lower Helderberg. 2. S. nia- 
garensis Hall, Niagara. 3. S. miagarensis var., Niagara. 4. S. niagarensis 
var. Niagara. 5. S. divolvis White and Whitfield, Lower Carboniferous. 6. 
S. ventricosus Conrad, Oriskany, 7. S. peoriensis McChesney, Upper Car- 
boniferous. 9. S. Zimeatus Conrad, Upper Helderberg. 10. S. lineatus var. 
. II. S. varians Hall, Upper Helderberg. 12. S. varians var., 
Upper Helderberg. 13. S. äneatus Conrad, Hamilton. 14. S. unisulc 
Conrad, Upper Helderberg. 15. S. furbinatus Hall, Upper Helderberg. 
16. S. turbinatus var. cochlea, Hamilton. 17. S. turbinatus,var. 18. S. 
expansus, Oriskany. 19. S. cyelostomus Hall, Niagara. 20. S. transversus 
all, Oriskany. 21. S. arenosus Conrad, Lower Helderberg. 22. S. matheri 
Hall, Oriskany. 23. S. andrewsi Hall, Oriskany. 
