Given the complete lack of provings of Cephalochordates, and the presence of only one trituration, any provings of Lancelets would be immensely beneficial. 
As the one trituration available comes from the Branchiostoma genus, of the Branchiostomidae, members of the genii Epigonichthys or Asymmetron of the Assymetronidae would be the most sensible targets. With two more provings or triturations we could easily cover all extant genii within this subphyla.
As well, triturations and provings of some of the Cephalochordate fossils such as Cathymyrus species might also prove illuminating.