▁most ▁important ▁species ▁from ▁a ▁bi ost rat ig raph ical ▁point ▁of ▁view . ▁This ▁s uture ▁type ▁is ▁characteristic ▁of ▁Jur ass ic ▁and ▁C ret ace ous ▁am m ono ids , ▁but ▁extends ▁back ▁all ▁the ▁way ▁to ▁the ▁Perm ian . ▁ ▁Si ph un cle ▁ ▁The ▁si ph un cle ▁in ▁most ▁am m ono ids ▁is ▁a ▁narrow ▁tub ular ▁structure ▁that ▁runs ▁along ▁the ▁shell ' s ▁outer ▁rim , ▁known ▁as ▁the ▁v enter , ▁connecting ▁the ▁ch amb ers ▁of ▁the ▁ph rag mo con e ▁to ▁the ▁body ▁or ▁living ▁chamber . ▁This ▁distingu ishes ▁them ▁from ▁living ▁na util o ides ▁( N aut ilus ▁and ▁All onaut ilus ) ▁and ▁typical ▁N aut il ida , ▁in ▁which ▁the ▁si ph un cle ▁runs ▁through ▁the ▁center ▁of ▁each ▁chamber . ▁However ▁the ▁very ▁earliest ▁na util oid s ▁from ▁the ▁Late ▁Camb rian ▁and ▁Ord ov ician ▁typically ▁had ▁vent ral ▁si ph un cles ▁like ▁am mon ites , ▁although ▁often ▁proportion ally ▁larger ▁and ▁more ▁internally ▁struct ured . ▁ ▁The ▁word ▁" si ph un cle " ▁comes ▁from ▁the ▁New ▁Latin ▁si ph un cul us , ▁meaning ▁" l ittle ▁si phon ". ▁ ▁Classification ▁ ▁Origin ating ▁from ▁within ▁the ▁b act rit oid ▁na util oid s , ▁the ▁am m ono id ▁ce phal op ods ▁first ▁appeared ▁in ▁the ▁Dev onian ▁( cir ca ▁ 4 0 9 ▁million