A study of over 500 rabbit skulls shows 2 distinct types, "dome" and "ridge." These result from the combined action of a group of unit factors, "reverse," "accessory" and "fused," the individual effects of which upon the size and shape of individual bones, suture pattern and upon the character. and angle of the bony union with the bregma are described. The 3 factors are inherited as distinct entities capable of being transmitted unaltered. In general, they are recessive to the corresponding normal condition, although there is some variation in expression and they can be recombined to form the parental types as well as new types capable of transmission.