angulation that gradually becomes stronger and bears nodules formed 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 probably 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 S. subspinosa, S. cincta, and some others of the group, a faint spiral cingulum appears half-way between the shoulder and the suture. This disappears after one or two volutions, but, together with the angled shoulder, it may represent the stage at which

Fig. 9.—a. Apex of Struthiolaria papulosa; × 6. b. Same; × 12. c. Struthiolaria subspinosa; × 3.

diverged the S. vermis group, with its bicarinate spire-whorls. This bicarination has practically disappeared from the early whorls of S. papulosa, but some specimens have a suggestion of it.

Traces 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—i.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 agreement in details of sculpture points rather to direct descent of the latter from the former.