THE AVONIAN OF THE AVON GORGE 
77 
be continuously connected in time. (An individual in which all 
the characters of a certain species are reproduced, but which 
came into existence long after that species had died' out, cannot 
be correctly assigned to the same species.) 
The whole assemblage, which is composed of the ancestors, 
descendants and collateral relatives of a given species, constitutes 
the gens of that species and is denoted by prefixing ‘ aff .’ to 
the specific name. Thus, Productus aff. semireticulatus denotes 
any member of the gens of which Prod, semireticulatus (Martin) 
is a well-known member. 
In the case of a long-lived gens, an early member usually 
differs sufficiently from a late one to constitute a new species. 
All such time- variants of a given gens are here referred to as 
mutations of the gens and the time at which a given 
mutation flourished is indicated by the addition of a zonal or 
subzonal symbol to the abbreviation ‘ mut.’ Thus, Spirifer 
aff. bisulcatus, mut. C. implies a species which existed at the 
time indicated by the zonal symbol C. (v.i.) and which is a 
member of the gens containing Spirifer bisulcatus, J. de C. 
Sow. 
Ahomoeomorph is a form which strongly resembles the 
species with which it is compared, although it is presumably not 
a member of the same gens. Homoeomorphy is indicated by 
inserting ‘ cf.’ before the specific name of the form with which 
comparison is made. Thus, Orthothetes cf. crenistria bears a 
strong resemblance to ‘ Spirifera ’ crenistria, Phillips (as typified 
by the specimen preserved in the Gilbertson Collection), but the 
two species are presumably not members of the. same gens since 
Phillip’s species appears to be a Derby a, whereas the form which 
is here denoted by Orthothetes cf. crenistria is a true Orthothetes. 
The Theory Stratigraphical Zones are subdivisions which succeed 
of Zoning each other always in a definite order ; they may be 
defined by the occurrence of a particular rock-type (‘ Lithozones ’), 
or by some faunal particularity (‘ Biozones ’). 
Lithozones, by their very nature, can only determine horizons 
