胎盤菊石

胎盤菊石（學名：），又名平盤菊石、餅菊石，是生存在晚白堊紀海洋中的一屬菊石。牠們可能是所有已知的菊石中游速最快的，透過噴水的動力來追逐獵物、逃避兩棲動物與魚類的攻擊。其化石在亞洲、歐洲和美洲都有發現。

 特徵 
胎盤菊石具有緊壓的、內旋的殼，腹狹窄，有散發光彩的厚實真珠質層。牠們最大可長到50公分長，其巨大的體型和流線型的殼體使牠們在水中能快速游動。

 物種 
Placenticeras bidorsatum 
Placenticeras cumminsi 
Placenticeras fritschi 
Placenticeras grossouvrei 
Placenticeras kolbajense 
Placenticeras maherndli 
Placenticeras mediasiaticum 
Placenticeras memoriaschloenbachi 
Placenticeras orbignyanum 
Placenticeras paraplanum 
Placenticeras placenta 
Placenticeras polyopsis 
Placenticeras pseudoplacenta 
Placenticeras semiornatum 
Placenticeras syrtale 
Placenticeras tamulicum 
Placenticeras vredenburgi 
Placenticeras whitfieldi 

 參考文獻 

 Arkell et al., 1957,  Mesozoic Ammonoidea, Treatise on Invertebrate Paleontology Part L.  Geological Soc. of America, Univ of Kansas Press. R.C. Moore, (Ed)
 W. A. Cobban and Hook, S. C. 1983 Mid-Cretaceous (Turonian) ammonite fauna from Fence Lake area of west-central New Mexico. Memoir 41, New Mexico Bureau of Mines&Mineral Resources, Socorro NM.
 W. A. Cobban and Hook, S. C. 1979, Collignoniceras woollgari wooollgari (Mantell) ammonite fauna from Upper Cretaceous of Western Interior, United States. Memoir 37, New Mexico Bureau of Mines&Mineral Resources, Socorro NM.
 Discovering Fossils: How to Find and Identify Remains of the Prehistoric Past (Fossils & Dinosaurs) by Frank A. Garcia, Donald S. Miller, and Jasper Burns

 外部連結 
 Placenticeras in the Paleobiology Database

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