' s ▁upper ▁surface ▁and ▁the ▁lower ▁giving ▁one ▁of ▁its ▁unders ide . ▁ ▁Od ont og ri ph us ▁was ▁a ▁flat - b od ied ▁animal ▁r anging ▁from ▁ ▁to ▁ ▁in ▁length , ▁with ▁parallel ▁sides ▁and ▁semi - circ ular ▁ends . ▁The ▁spec im ens ▁exam ined ▁by ▁Car on , ▁Sch el tem a ▁et ▁al . ▁( 2 0 0 6 ) ▁had ▁the ▁same ▁ratio ▁of ▁length ▁to ▁width ▁ir res pective ▁of ▁size . ▁The ▁body ▁out lines ▁are ▁bil ater ally ▁symmet rical ▁in ▁all ▁fairly ▁complete ▁spec im ens , ▁even ▁those ▁in ▁which ▁internal ▁features ▁were ▁preserved ▁asym met r ically . ▁Car on , ▁Sch el tem a ▁et ▁al . ▁( 2 0 0 6 ) ▁interpreted ▁this ▁as ▁evidence ▁that ▁the ▁animals ▁had ▁on ▁their ▁back s ▁" sh ells " ▁that ▁were ▁rig id ▁enough ▁to ▁resist ▁whatever ▁str esses ▁dist orted ▁the ▁internal ▁features , ▁but ▁were ▁not ▁t ough ▁enough ▁to ▁be ▁preserved ▁by ▁foss il ization ▁– ▁similar , ▁for ▁example , ▁to ▁finger ▁n ails . ▁Rel atively ▁broad ▁wr ink les , ▁parallel ▁to ▁each ▁other ▁and ▁usually ▁straight , ▁run ▁across ▁the ▁central ▁region ▁of ▁the ▁body ▁in ▁some ▁spec im ens . ▁ ▁Car on , ▁Sch el tem a ▁et ▁al . ▁( 2 0 0 6 ) ▁found ▁evidence ▁of ▁a ▁circular ▁mouth ▁on ▁the ▁unders ide , ▁with ▁two ▁and ▁occasionally ▁three ▁to oth -