Well its a bit longwinded but The Corylus group on
http://www.ucl.ac.uk/archaeology/research/profiles
/smason/smcoryl2.htm
say
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The Kentish Cob is a variety with a large ovoid-oblong nut, introduced to cultivation in the early nineteenth century. The origin of cultivated hazelnuts is confused (and confusing) - cob nuts, with the husk shorter than the nut, probably derive largely from C. avellana, while filberts (of which the Kentish Cob is, even more confusingly, an example), probably derive largely from C. maxima, native to Southeast Europe and northern Turkey. According to Daniel Zohary (personal communication) it is now being concluded that the two 'species' are in fact two ends of a geographical cline. It is also likely that much of the 'wild' C. avellana growing in the area contains genes introgressed from cultivated varieties (and this introgressed form is spreading as so-called 'native' stock used for landscaping is raised by seed from Kentish Cob.