Here is the information from my book (some things must have missed copy pasting from Pagemaker)
Names of Hybrids
Hybridity is indicated by the use of the multiplication sign, or by the addition of the prefix ‘notho-’ to the term denoting the rank of the taxon, the principal ranks being nothogenus and nothospecies. A hybrid between named taxa may be indicated by placing the multiplication sign between the names of the taxa; the whole expression is then called a hybrid formula:
1. Agrostis X Polypogon
2. Agrostis stolonifera X Polypogon monspeliensis
3. Salix aurita X S. caprea
It is usually preferable to place the names or epithets in a formula in alphabetical order. The direction of a cross may be indicated by including the sexual symbols ( : female; : male) in the formula, or by placing the female parent first. If a non-alphabetical sequence is used, its basis should be clearly indicated.
A hybrid may either be interspecific (between two species belonging to the same genus) or intergeneric (between two species belonging to two different genera). A binary name may be given to the interspecific hybrid or nothospecies (if it is self-perpetuating and/or reproductively isolated) by placing the cross sign (if mathematical sign is available it should be placed immediately before the specific epithet, otherwise ‘x’ in lower case may be used with a gap) before the specific epithet as in the following cases (hybrid formula may be added within the parentheses if the parents are established):
1. Salix x capreola (S. aurita X S. caprea)
or Salix Xcapreola (S. aurita X S. caprea)
2. Rosa x odorata (R. chinensis X R. gigantea)
or Rosa Xodorata (R. chinensis X R. gigantea)
The variants of interspecific hybrids are named nothosubspecies and nothovarieties, e.g. Salix rubens nothovar. basfordiana.
For an intergeneric hybrid, if given a distinct generic name, the name is formed as a condensed formula by using the first part (or whole) of one parental genus and last part (or whole) of another genus (but not the whole of both genera). A cross sign is placed before the generic name of the hybrid, e.g. XTriticosecale (or x Triticosecale) from Triticum and Secale, XPyronia (or x Pyronia) from Pyrus and Cydonia, and Agropogon from Agrostis and Polypogon.. The names may be written as under:
1. XTriticosecale (Triticum X Secale)
2. XPyronia (Pyrus X Cydonia)
The nothogeneric name of an intergeneric hybrid derived from four or more genera is formed from the name of a person to which is added the termination -ara; no such name may exceed eight syllables. Such a name is regarded as a condensed formula:
XPotinara (Brassavola X Cattleya X Laelia
The nothogeneric name of a trigeneric hybrid is either: (a) a condensed formula in which the three names adopted for the parental genera are combined into a single word not exceeding eight syllables, using the whole or first part of one, followed by the whole or any part of another, followed by the whole or last part of the third (but not the whole of all three) and, optionally, one or two connecting vowels; or (b) a name formed like that of a nothogenus derived from four or more genera, i.e., from a personal name to which is added the termination -ara:
XSophrolaeliocattleya (Sophronitis X Laelia X Cattleya)
When a nothogeneric name is formed from the name of a person by adding the termination -ara, that person should preferably be a collector, grower, or student of the group.
A binomial for the intergeneric hybrid may similarly be written as under:
XAgropogon lutosus (Agrostis stolonifera X Polypogon monspeliensis)
It is important to note that a binomial for an interspecific hybrid has a cross before the specific epithet, whereas in an intergeneric hybrid, it is before the generic name. Since the names of nothogenera and nothotaxa with the rank of a subdivision of a genus are condensed formulae or treated as such, they do not have types.
Since the name of a nothotaxon at the rank of species or below has a type, statements of parentage play a secondary part in determining the application of the name.
The grafts between two species are indicated by a plus sign between two grafted species as, for example, Rosa webbiana + R. floribunda.