DEVONIAN FOSSILS 103 
These specimens have the same long and well-defined ventral 
muscle field, although not quite so prominent as in P. affinalata. 
The ridges dividing the diductors into three bundles are present, 
but again not quite so prominently. Also, the whole interior of the 
valve is covered with fine papillae, but this secondary deposition 
is not sufficiently thick to completely mask the external ornament 
as is the case with the Sandy’s Creek fossils. In short, the same 
structures are present in the specimens from the two localities, 
but their development is less pronounced in the Mooroolbark form. 
One is presented with the problem as to whether these differences 
are genotypic or phenotypic, due to inherent constitution or 
merely to facies effects or differences in degree of maturity. An 
attempt was made to solve this by studying the young forms of 
P. affinalata preserved on the same slab as the holotype. It was 
noted that in specimens half the size of the holotype that the same 
strong papillosity is present and extends right to the edge of the 
shell. It is thus clear that the extent of the internal ornament is 
not affected by degree of maturity. The same applies to the 
general definition of the muscle field. However, the median sep- 
tum and ridges dividing the ventral diductors into bundles are 
very indistinct in the young specimens; these therefore are 
features that vary with degree of maturity. As there are no 
major facies differences between the Sandy’s Creek beds and the 
Mooroolbark ones in which the fossils under discussion were 
found, it may be inferred that the difference in degree of internal 
ornament is a genotypic and not a phenotypic one. The most 
developed of the specimens from Mooroolbark is not nearly as 
advanced as the well-developed specimens from Sandy’s Creek. 
Variant Form. Specimen 27,229A from locality G 24 preserves 
a ventral valve of Protoleptostrophia which varies from P. affina- 
lata in that the teeth occupy the whole of the ventral interarea, 
and not just the anterior part of it. This is the only specimen 
noted in the collection with this variation. 
Genus HIPPARIONYX Vanuxem, 1842 
Hipparionyx major sp. Nov. 
РІ. II, Figs. 1-3, 8. 
Type Material. H olotype consisting of the steinkern of a ven- 
tral and a dorsal valve lying with hingelines together, flat open 
(specimens 27,177 and 97,179 glued together) and the external 
mould of same (specimens 27,178 and 27,201 glued together) from 
locality G22. As the steinkern and external mould are impres- 
