352 Notes and Comments. 
list of species presents some features of considerable interest 
bearing on the value of certain types as zonal indices. Reference 
is made to Vaughan’s paper on the Bristol area, where it is 
indicated that amongst the brachiopod groups confined to the 
Tournaisian in that area are the following: Productus cf. 
martin ; Leptena analoga; Schizophoria resupinata; Rhipi- 
domella aff. michelint; Spiriferina octoplicata ; Syringothyris 
cuspidata. Two of these, it is noted, Sfriferina octoplicata 
and Schizophoria resupinata, are sub-zonal indices, and each 
with its maximum in its sub-zone._ The list of Castleton forms 
from well up in D, now presented, includes all the above- 
mentioned brachiopod groups. Syringothyris cuspidata and 
Spiriferina octoplicata are fairly abundant at both Treak 
Cliff and Peakshill, Schizophoria resupinata is extremely abund- 
ant at both places, Leptena analoga is abundant whilst Pyvo- 
ductus cf. martint and Rhipidomella michelin are rare. 
AT TREAK CLIFF AND PEAKSHILL, CASTLETON, DERBYSHIRE. 
Passing to the coral fauna, the genus Zaphrentis appears 
in the Castleton list, 1.e., one of the two genera of corals con- 
fined to the Tournaisian in the Bristol area and not extending 
into the Viséan. The genus, though not*very abundant, is 
represented by several species. In addition, the genera 
Michelinia and Amplexus, characteristic of the Upper Tournais- 
ian of Bristol, but possibly extending into the base of the Viséan 
are cited in the Castleton list, Michelinia glomerata being fairly 
aboundant at Peakshill, and Amplexus coralloides is found at 
Treak Cliff, but is extremely rare. 
ZONAL DETERMINATION. 
These facts lead to a consideration as to how far the types 
mentioned are of value in zonal determinations. If any one of 
them, as recorded from Castleton, be regarded as representing 
exactly the same form as that recorded from the Bristol area, 
then its value as one of a number of index fossils of a zone 
becomes negligible. Examples are cited in the cases of Spiri- 
ferina octoplicata and Schizophoria resupinata. 1f the Castleton 
forms of D? horizon agree in identity with the Bristol types of 
Kk? and Z? respectively, then these two types become worthless 
as sub-zonal indices. It was pointed out that, even allowing of 
the rather unlikely possibility that in all the cases cited the 
Castleton specimens represented mutational forms of the 
Bristol species, the real difficulty as to their zonal value is not 
overcome, since the line of demarcation between mutations is 
more or less arbitrary and there is still a considerable field of dis- 
cussion as to what constitutes a ‘mutation.’ It appears probable 
that any system of zonal indices can be of local value only, as for 
example in the application of the Bristol zonal indices within 
the Bristol area, and cannot be of any general application. 
Cad 
Naturalist, 
