Why some scientific names end with "-i" and some with "-ii" and it is both correct?

The Endings "-i" and "-ii" in Species Epithets

The distinction between the "-i" and "-ii" endings in zoological patronyms arises from Latin grammatical structure and rules regarding the treatment of names. The usage of these endings reflects the gender and form of the original Latinization of the individual's name.

The endings "-i" and "-ii" apply exclusively to the singular genitive (possessive) form of nouns—specifically, names.

A patronym is generally defined as a name derived from the name of a father or an ancestor, typically used to signify lineage or heritage. Etymologically, the term "patronym" comes from the Greek word "patēr" (πατήρ, meaning "father") and the suffix "-onym," which originates from the Greek "onuma" (ὄνομα, meaning "name"). Thus, it literally translates to "father's name." This is a strict interpretation. In a broader context, a zoological species name can be derived from any person's name (family name or given name) or, in some cases, even from a geographic name, which is also allowed to be formed in the genitive case.

In extreme cases, patronyms and geographic names might be indistinguishable: in the family names of nobles, the family name often refers to the location of origin and is preceded, for example, by "von" in German, which translates to "of" or "from" in English. The equivalent in French and Spanish is "de," in Italian "di," and in Czech "z," etc. An example might be Bothrops neuwiedi, after Prince Alexander Philipp Maximilian von Wied-Neuwied. Or in the family Chamaeleonidae Rhampholeon tiburyi (after Dr. Colin Tilbury), Chamaeleo necasi (after Dr. Petr Necas), Trioceros werneri (after Dr. Franz Werner), Furcifer nicosiai (after H.E. Guido Nicosia), Brookesia peyrierasi (after Andre  Peyieras) etc.

In zoology and botany, patronyms may be used to name species after individuals—often scientists or first finders—who contribute to the field. However, it is at the discretion of the author(s) to choose any person and its name for whatever reason. The name usually honors the individual being commemorated.

Latinization of Names

In zoological naming, patronyms are formed by taking the surname of a person and adapting it to fit Latin grammatical rules.

An astute reader may point out that e.g. in the case of Trioceros hoehnelii, named after the brave African explorer of Austrian origin, Rear Admiral Ludwig Ritter von Höhnel, the name should technically be "hoehneli" but is, in fact, "hoehnelii." Yes, this is correct.

There is a way to make Ludwig Ritter von Höhnel's name conveniently end in "-ius." The trick involves somewhat forcefully Latinizing his name (which must be Latinized anyway, as the letter "ö" does not exist in Latin and needs to be replaced by "oe" through phonetic transliteration, or by simply omitting the diacritical mark of the umlaut).

So, what did Dr. Franz Steindachner do while authoring the description of Chamaelon hoehnelii (besides making a typo in Chamaelon instead of Chamaeleon, he retained, at that time in 1891, the capital letter "H" and the letter "ö," which were converted to a small "h" and "oe" later). He forcefully Latinized the name to "Höhnelius." From this, forming the genitive is straightforward: "Höhnelii," which was later modified to "hoehnelii." Moreover, there are several names with similar adaptations.

In the family Chamaeleonidae the species names ending with "-i" and "-ii" are as follows: 

Ending "-i"

• Calumma parsonii
  
• Chamaeleo dilepis petersii
  
• Chamaeleo ruspolii 
  
• Furcifer willsii
  
• Rhampholeon beraduccii
  
• Rieppeleon kerstenii kerstenii 
  
• Rieppeleon kerstenii robecchii
  
• Trioceros hoehnelii 
  
• Trioceros jacksonii