Marwick.—The Struthiolariidae. 167 
angulation that gradually becomes stronger and bears nodules fórmed by 
the intersection of the growth-lines. On later whorls these nodules become 
more prominent and farther apart, finally developing into prominent tubercles, 
and numerous secondary endogeneous spirals appear. The stage of curved 
axial ribs so characteristic of Monalaria is not represented, so this is pro- 
bably a case of lipopalingenesis, or the dropping of an ancestral stage in 
the ontogeny of a specialized group (Grabau, 1904, p. 3; Trueman, 1922, 
p. 141). 
About the third conch-whorl of 8. subspinosa, S. cincta, and some 
others of the group, a faint spiral cingulum appears half-way between the 
shoulder and the suture. s disappears after one or two volutions, but, 
together with the angled shoulder, it may represent the stage at which 
Fu. c Apex of Struthiolaria у; 6. b. Same; x 12. с. Struthiolaria 
pinosa ; Lx 3. 
diverged the S. vermis group, with its bicarinate spire-whorls. This bicari- 
nation has practically disappeared from the early whorls of S. papulosa, 
but some specimens have a suggestion of it. 
races of the double lower keel of M. concinna linger in some specimens 
of S. subspinosa, but in the other Miocene species, such as S. spinosa, this 
keel is single, while in the Pliocene and Recent S. papulosa it has disappeared, 
leaving only one angulation—;.e., at the shoulder of the body-whorl. 
More profound changes from the Monalaria stage are to be seen in the 
curved columella, and the appearance of a second angulation on the outer 
lip, opposite the posterior keel (or shoulder-angle). Indeed, these features 
may indicate- that Struthiolaria s. str. did not descend through Monalaria, 
but that the two are independent branches of an earlier convex- whorled 
ancestor. This would mean that the body-whorls of M. concinna and 
S. subspinosa are parallel developments, but their close цэм, in details 
of sculpture points rather to direct descent of the latter from the former. 
