Shell ultrastructure of the Atlantidae (Heteropoda, Mesogastropoda) Oxygyrus and Protatlanta, with comments on Atlanta inclinata. Bulletin of the AMNH ; v. 157, article 4

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New York : American Museum of Natural History
"The purpose of the present study is to describe and interpret the wall ultrastructure of three atlantid heteropod species for phylogenetic and taxonomic analysis; they are Oxygyrus keraudreni, Protatlanta souleyeti, and Atlanta inclinata found in shallow waters of subtropic and tropic oceanic regions. Whereas most other members of predominantly thick-shelled, benthic mesogastropod families have three or more calcified crossed-lamellar layers termed here 'normal,' the exceedingly thin-walled planktonic atlantids have highly modified, crossed-lamellar layers termed here 'Type-2.' This 'Type-2' structure was perhaps derived from the normal mesogastropod type structure as a result of thinness of the wall, as this structure has been observed in unrelated thin-shelled taxa, such as pteropods (opisthobranchs), scissurellids (archaeogastropods), and early embryonic stages of fissurellids (archaeogastropods). An important variation is that both the first-order lamellae and the second-order rods may be rigid 'Type-2,' sigmoidal, or curved so that a layer may resemble helical structure. A new type of prismatic layer, asymmetric prismatic, is described and distinguished from simple prismatic, apertural prismatic, and myostracal structure. Variations of ultrastructure within samples of Oxygyrus keraudreni from the same population were found to have variant patterns identical with those of distant populations. Ornamented and unornamented embryonic Protatlanta souleyeti were found within populations in about the same ratio. The wall structure among Oxygyrus keraudreni, Protatlanta souleyeti, and Atlanta inclinata is distinct, with variations that can be used to distinguish each species. Embryonic variations of ultrastructure are characterized by changing thickness of wall structure during growth, whereas adult variations of ultrastructure are characterized by morphologic changes within a specific wall unit type. A study of heteropod material from plankton tows and bottom dredges failed to establish any recognizable difference in alteration of ultrastructure by selective solution in deep sea sediments. Both types of samples displayed about the same degree of alteration, implying that factors either biological or chemical are operating on individuals even during their life cycle"--P. 267.
p. 265-310 : ill. ; 26 cm.
Includes bibliographical references (p. 309-310).