. ▁The ▁six ▁post - n uc lear ▁wh or ls ▁are ▁fl atten ed . ▁They ▁are ▁marked ▁by ▁three ▁equal ▁and ▁equally ▁spac ed , ▁strong , ▁spir al ▁ke els , ▁of ▁which ▁one ▁is ▁at ▁the ▁sum mit ▁and ▁another ▁about ▁as ▁far ▁above ▁the ▁peri ph ery ▁as ▁the ▁space ▁which ▁separ ates ▁it ▁from ▁the ▁median ▁ke el . ▁The ▁spaces ▁between ▁the ▁spir al ▁ke els ▁are ▁deep , ▁rounded ▁gro ov es , ▁almost ▁as ▁wide ▁as ▁the ▁ke els ▁and ▁crossed ▁by ▁numerous , ▁sl ender , ▁ax ial ▁threads . ▁ ▁The ▁s ut ures ▁are ▁deeply ▁ch anne led . ▁These ▁channels ▁ ▁are ▁a ▁little ▁more ▁prof ound ▁than ▁those ▁between ▁the ▁ke els . ▁The ▁peri ph ery ▁of ▁the ▁body ▁wh or l ▁is ▁marked ▁by ▁a ▁sul cus . ▁The ▁base ▁of ▁the ▁shell ▁is ▁short , ▁well ▁rounded , ▁and ▁marked ▁by ▁four ▁sub equal ▁and ▁sub equ ally ▁spac ed , ▁spir al ▁c ords , ▁the ▁spaces ▁between ▁which ▁appear ▁as ▁rather ▁broad ▁sul ci ▁and ▁are ▁crossed ▁by ▁sl ender ▁ax ial ▁threads . ▁The ▁ov ate ▁a pert ure ▁is ▁small ▁and ▁very ▁ob lique . ▁The ▁posterior ▁angle is ▁ ▁obt use . ▁The ▁outer ▁lip ▁is ▁thin , ▁showing ▁the ▁external ▁sculpt ure ▁within . ▁The ▁inner ▁lip ▁is ▁short , ▁cur ved , ▁slightly ▁revol ute , ▁provided ▁with ▁a ▁strong ▁fol d ▁at ▁its ▁insert ion . ▁The ▁par