▁P iers al oc eras ▁Te ich ert ▁▁ 1 9 3 0 : ▁like ▁G rac il oc eras ▁but ▁larger , ▁not ▁distinct ly ▁com pressed , ▁si ph un cle ▁well ▁removed ▁from ▁v enter , ▁surface ▁with ▁weak ▁longitud inal ▁rib s ▁crossed ▁by ▁thick , ▁r ipp le - sh aped ▁fl utt ings . ▁U ▁Ord ▁ ▁E ▁Eur . ▁( S weet ▁K 3 8 3 ) ▁▁ ▁Ring oc eras ▁ ▁Str and ▁▁ 1 9 3 4 : ▁small , ▁strongly ▁cur ved , ▁de pressed ; ▁surface ▁with ▁longitud inal ▁rib s ▁and ▁fine , ▁trans verse ▁st ria e ; ▁si ph un cle ▁sub vent ral ; ▁U ▁Ord ▁ ▁N ▁Eur . ▁( S weet ▁K 3 8 3 )& ▁▁ ▁K und oc eras ▁Kr ö ger ▁ 2 0 0 9 : ▁a ▁ ▁g rac il oc er at id ▁from ▁the ▁Middle ▁Ord ov ician ▁of ▁the ▁Balt ic ▁region ▁( K r ö ger , ▁ ▁Z hang , ▁and ▁ ▁Is ak ar . ▁ 2 0 0 9 ) ▁ ▁References ▁ ▁F lower , R . H ▁ 1 9 5 0 ▁in ▁F lower ▁and ▁K um mel , ▁A ▁Classification ▁of ▁the ▁N aut ilo idea , ▁Journal ▁of ▁Pale ont ology , ▁Vol ▁ 2 4 , ▁no . 5 , ▁pp 6 0 4 – 6 1 6 , ▁Sept ▁ 1 9 5 0 ▁ ▁S weet