Bulletins of American paleontology . ome established in Chile. Timesof dispersal of Mediargo and Fusitriton are indefinite, the ages ofmany specimens being given as Oligocene or Miocene. The formergenus is slightly older and may have given rise to the latter, as itappears lower in the lower Miocene section in Kern County, Cali-fornia. The oldest species of Fusitriton is F. dilleri, from which probably evolved, although a more thorough evalua-tion of forms from the Miocene Yakataga Formation of Alaskaand from the Japanese Miocene could modify the simple relation-ship shown. Middle
Bulletins of American paleontology . ome established in Chile. Timesof dispersal of Mediargo and Fusitriton are indefinite, the ages ofmany specimens being given as Oligocene or Miocene. The formergenus is slightly older and may have given rise to the latter, as itappears lower in the lower Miocene section in Kern County, Cali-fornia. The oldest species of Fusitriton is F. dilleri, from which probably evolved, although a more thorough evalua-tion of forms from the Miocene Yakataga Formation of Alaskaand from the Japanese Miocene could modify the simple relation-ship shown. Middle to late Miocene specimens of Mediargo mathewsoniiand M. mediocris are completely gradational forms. Almost identicalforms of M. mathewsonii were widespread in the early and middleMiocene between Kern County, California, Vancouver Island,British Columbia, northern Honshu, and northeastern younger species has not been reported from the westernPacific but apparently evolved in the late Miocene to early Plio- 526 Bulletin 254. Text-figure 14. — Possible phylogenetic relationships between Argobucci-num, Fusitriton, Priene, and Mcdiargo. 1, Fusitriton galea; 2, F. canccllatusmurrayi; 3, F. rctiolus; 4, Argobuccmum argus; 5, A. proditor; 6, A. tumidum;7, A. tristaticnse; 8, Pricnr rude. Cymatiid Gastropods: Smith 527 cene from M. mathcicsonii in the area between Washington andcentral Cahfornia. M. mcdiocris is rare in many of the laterPliocene formations of California and commonest in the SantaMaria District of southern California, where a more favorablemode of preservation in tar seeps may account for its species ranged into the late Pliocene and became extinctbefore the Pleistocene. The Miocene record of Argobuccmiim in Chile rests on onespecimen from the Navidad Formation; the small number of col-lections may account for its apparent rarity in Plio-Pleistocene as-semblages. Its dispersal to South Africa and New Zealand occurredbefore the end of the Pleist
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