Complex sentence: They were discovered by Thorold Gosset, although only the demipenteract (and lower regular forms) were included in his regular and semiregular polytope list since he defined semiregular to mean only regular polytope facets could be used.
Split into simpler sentences: They were discovered by Thorold Gosset, although only the demipenteract (and lower regular forms) were included in his regular and semiregular polytope list. Higher forms were excluded because defined semiregular to mean only regular polytope facets could be used.