276 
THE MIDDLE LIAS OP NORTHAMPTONSHIRE. 
latter ammonite is abundant in some of the lower beds, and 
passes upwards very near to, but just sliortof the “Margaritatus” 
Zone. Ammonites margaritatus is most common in bed “ F,” and 
is very variable in form; the varieties A. gibbosus (SchL), A 
spinosus (Querist .), and A. sabnodosus ( Y. £ B.), and A. depressus 
(Quenst .) being all probably represented. Ammonites spiuatus 
is decidedly rare in Northamptonshire, and appears to be met 
with only near the top of the Rock-bed. Ammonites acutus is 
very abundant in, and characteristic of the Transition-bed, 
but it is found occasionally just below or just above this 
zone. It is not uncommon to find forms having decided 
transitional characters leading to other members of the 
“ falcifer” group, which are common in the Upper Lias. A 
flattening of the sides, brought about, not by a thinning of 
the shell, but by a filling out of the inner and outer edges of the 
whorls, with a consequently more abrupt fall into the umbi¬ 
licus, and the development of a more distinct ribbing, seem 
to be changes which take place contemporaneously. It may 
be of interest to note here that the average specimens found 
at Tilton, in Leicestershire, are larger than the average ones 
found in Northamptonshire, and that the changes noted 
above may be more often observed in these larger ones. 
I have already called attention to the abundance of 
Gasteropods in the Transition-bed, and the table of fossils 
well shows it; but the upper bed of the “ Margaritatus ” 
Zone —bed “ F ”—sometimes contains a large number of 
specimens, although the number of distinct species is small. 
Some recent work has revealed to me the fact that they are 
much more abundant in this bed than I had before suspected. 
The list of fossils does not show a very marked difference 
between the “Margaritatus” and “ Spmatus” Zones as regards 
the distribution of the Lamellibrancliiata, yet the former 
zone does contain a very much greater number of these 
than the latter. Speaking generally, Pectens are the 
only Lamellibranchs which are very common in the “ S/dn- 
atus ” Zone. Hinnites and Limas are practically not found 
below this zone, but they are not very common in it. 
Pecten liasinus (Nyst .) occurs pretty evenly distributed in 
all the three zones, and the same remark applies to Pecten 
cequivalvis, though the variety Pecten dentatus (Sow.) seems 
particularly characteristic of the Rock-bed “ 13.” 
One thing which can scarcely fail to attract the attention 
of anyone studying beds of the “ Margaritatus ” and “ Spin- 
atus ” Zones is the large number of Cardinias in certain 
restricted areas. Cardinia antiqua is fairly abundant in the 
lower beds in a good number of places, but more so at Milton 
