† protosiphonulan.ZoologyObsoleterare (A. Hyatt's name for) an early stage in the embryonic development of certain cephalopod molluscs.
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1893 A. Hyatt in Proc. Boston Soc. Nat. Hist. 103 I first described this substage among the nautiloids under the descriptive name of the ‘asiphonula’ but have since substituted the term, Protosiphonula.