The group of species with dissected leaves in eastern Asia including Japan has many problems of taxonomy.
In general (in Japan) the recogized taxa are as outlined below. But this view is provisional and not the only one.

Viola chaerophylloides f. sieboldiana   Japan

Viola chaerophylloides f. chaerophylloides  Japan(rare), Korea, China(Manshuria), Russia(Pirmorie)

Viola eizanensis Japan

Viola dactylloides  China(Manshuria), Russia(Pirmorie)

Viola dissecta China, Russia(Pirmorie)

V. chaerophylloides

There are two types of this species. One with deeply dissected leaves divided into linear lobes and another with comparatively wide lobes. The former is f. sieboldiana  and the latter is f. chaerophylloides. The former has wider distribution (from Russian Pirmorie, China, Korea and Japan) than the latter (Korea, China (Manshuria), Japan (rare)).
In Japan almost all occurrences of V. chaerophylloides are f. sieboldiana. Only in a limited area (Tsushima Island) we can see f. chaerophylloides. But there is some debate whether the plants on Tsushima and those in the other countries are the same.
And botanists in both China and Russia ordinarily donft recognize taxa below species level in V. chaerophylloides.
I have never seen this species on the continent except in a herbarium, so I donft have enough knowledge to comment further, especially about f. chaerophylloides.

Viola eizanensis

This species is endemic in Japan.  Its habitat is shadier and more moist than that of V. chaerophylloides.  Ordinarily these two species select different habitats to each other. V. chaerophylloides likes dry meadow or sparse woods in Japan. 
Viola eizanensis is more common than V. chaerophylloides in Japan.  These two species have almost no barriers to reproduction.  If they cross with each other, their F1 has almost complete ability to reproduce.  However these two species are recognizable as separate species because their habitats are considerably different and because of their form in general.
However, in rare cases, colonies of F1 or intermediates between these two species are found.

Viola dissecta

This species does not occur in Japan.  I have seen living plants only once and one individual in Pirmorie, Russia.  Although this species is similar to V. eizanensis, there are some differences in the shape of the leaves.  The main point is the angle of the leaf base. In the case of V. dissecta,  the petiole and base of leaf is vertical and in the case of V. eizanensis it is less than vertical.
The habitat of V. dissecta I saw in Russia was dry meadow - where V. eizanensis could never live.

Viola dactylloides 

This species has several distinguishing points of identification compared with other dissected-leaf species. One of these is the leaf is divided into five parts completely; also, the lobes are not deeply dissected.  This species does not occur in Japan. I have only seen a few individuals of this species, in Pirmorie Russia, and only once.

All rights reserved Masashi Igari 2001