I'm going to take a stab at the Xenophora question, but bear in mind I
am so NOT qualified. But until somebody more qualified logs on, here
goes

Okay, by genus;

Xenophora conchyliophora
Xenophora conchyliophora f. robusta
Xenophora senegalensis
Xenophora crispa
Xenophora neozelanica  neozelanica
Xenophora neozelanica kermadecensis
Xenophoraperonica kondoi
Xenophoratenuis
Xenophora flindersi flindersi
Xenophora pallidula
Xenophora corrugata
Xenophora granulosa
Xenophora mekranensis konoi
Xenophora peroniana peroniana
Xenophora cerea
Xenophora japonica
Xenophora solarioides solarioides
Xenophora granulosa

Stellaria solaris
Stellaria testigera digitata
Stellaria testigera profunda
Stellaria gigantea
Stellaria chinensis chinensis
Stellaria lamberti

Onustus caribaeus
Onustus longleyi
Onustus exutus
Onustus indicus

As for how many species, I think it depends who's counting. Is robusta
a subspecies of conchyliophora or a separate one? Beats me. The
acquisitive little gastropods worry not a bit about what they are. They
just crawl along looking for treasures to collect.