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Diverse lower Miocene to Pleistocene silicoflagellate assemblages occur at Deep Sea Drilling Project Site 495, but many samples are dominated by one or two taxa. Low-latitude zonation can be applied throughout. Cool-indicating Distephanus speculum s. ampl. is only abundant in the upper Miocene; however, relative paleotemperature values (Ts) suggest temperature extremes in the lower Miocene similar to those in the upper Miocene. The lower upper Miocene appears to be thinned or missing, because no Dictyocha brevispina Zone assemblages are identified. The lower Miocene assemblages of the Naviculopsis ponticula Zone are an important silicoflagellate reference because of several new taxa and associations. New taxa and recombinations described herein include: Corbisema triacantha var. nuda n. var., Dictyocha angulata n. sp., D. delicata n. comb., D. delicata var. bisecta n. var., D. longa n. sp., D. longa var.paxilla n. var., D. ornata ornata n. comb., D. ornata africana n. subsp., D. subaculeata n. comb., Distephanus crux parvus n. comb., D. crux scutulatusn. subsp., D. polyactis var. literatusn. var.,Zλ speculumpatulus n. subsp., Naviculopsis contraria n. sp., N. lacrima n. sp., N. lata var. obliqua n. var., N. ponticula spinosa n. subsp. INTRODUCTION Deep Sea Drilling Project Leg 67 cored a transect of seven sites across the Middle America Trench south of Guatemala to study tectonic accretion and imbrication of strata at a long-active convergent plate margin. Site 495 (12°29.78'N, 91°02.26'W, depth, 4140 m) is the distal oceanic reference site on the Cocos Plate, just 22 km seaward of the trench axis, and 1925 meters above the trench floor. Hole 495 was continuously cored from 19 to 446.5 meters sub-bottom depth, recovering basalt in Cores 46 to 49, lower and middle Miocene coccolithic ooze-chalk in Cores 20 to 45, and middle Miocene to Pleistocene diatomaceous muds in Cores 1 to 19. Silicoflagellates are fairly common and widely distributed through both sedimentary lithologies, when acid residue preparations are examined. Zonation, correlation, and relative paleotemperature index (7s) for the silicoflagellate assemblages are discussed. The systematic paleontology is divided into two sections—previously cited taxonomy and new taxonomy. Several of the new taxa are especially abundant, suggesting that upwelling might have magnified their importance and that the Miocene silicoflagellate succession in the Pacific is still incompletely known. SILICOFLAGELLATE ZONATION The silicoflagellate zonation employed for Site 495 is similar to that developed for nearby DSDP Leg 54 (Bukry, 1980b) and Leg 63 (Bukry, 1981), and is defined in Bukry (in press). Brief characterizations of the zones and subzones cited for Hole 495 (Figs. 1, 2) follow. Aubouin, J., von Huene, R., et al., [nil. Repts. DSDP, 67: Washington (U.S. Govt. Printing Office). Dictyocha aculeata Zone This upper Pleistocene zone is identified at Site 495 by the high concentration of Dictyocha sp. aff. D. aculeata (20%) and D. subaculeata (56%), with no specimens of Mesocena quadrangula. Dictyocha delicata Subzone This subzone is the upper unit of the Dictyocha stapedia Zone. It is mainly lower Pleistocene. The predominance of Dictyocha delicata (12%) over Mesocena quadrangula (6%), with Octactis pulchra (1%) present, is used to identify the subzone at Site 495. Dictyocha ornata Subzone This mainly upper Pliocene subzone is identified by the presence of Dictyocha ornata africana or D. ornata ornata and is the lower unit of the Dictyocha stapedia Zone. Although D. ornata africana and related forms occur only in Core 9, the high 27% abundance and conjunction with D. perlaevis flexatella (2%) are typical for the subzone. Curiously, no D. stapedia stapedia are present. Dictyocha stapedia Zone Some of the upper Pliocene assemblages lack the common subzonal guide taxa Dictyocha delicata or D. ornata. These assemblages are assigned to the undivided Dictyocha stapedia Zone on the basis of the abundant D. stapedia stapedia. Dictyocha fibula Zone Dictyocha fibula s. ampl. and D. longa predominate in the Dictyocha populations over asperoid taxa, below the range of D. stapedia stapedia. This indicates the Dictyocha fibula Zone of the upper Miocene and lower Pliocene.

Cenozoic Silicoflagellates from offshore Guatemala, Deep Sea Drilling Project Site 495