124 J. S. GARDNER ON BRITISH CRETACEOUS NUCULIDZ. 
situated about one fourth the length of the shell from its anterior 
margin, and the lunule is usually ill defined, and the test and lips 
smooth. In the subangular, or trigonal division the umbos are 
situated further forward, generally at one fifth of the whole iength 
from the anterior margin, the lunule is a distinctly defined region, 
and the species are generally sculptural and the lip toothed. For 
this division I propose to adopt D’Orbigny’s term Pectinatz, a name 
recalling the prevailing sculpture and the most typical form. The 
characters implied by the proposed terms will most readily be recol- 
lected if they are associated with the two most classic and best- 
known fossil Wucule, NV. ovata, Mant., and NV. pectinata, Sby., of the 
Folkestone Gault. The short side to which the umbones are turned 
is termed, and perhaps correctly, the posterior side by some authors ; 
but in the present description the short side on which the lunule 
occurs is referred to as the anterior, the long side as the posterior, 
and the side opposite the umbo as the inferior margin. Length is 
the extreme measurement from anterior to posterior margin; height 
the distance from the inferior margin in a straight line to the top of 
the shell above the umbo; and breadth or depth the distance from 
the chief projection of one valve through to the chief projection of 
the other valve. 
For the sake of brevity I have stated the chief characteristics 
of each subdivision but once, and they are not recapitulated in 
the descriptions of species. I have also tabulated together the 
dimensions of the species and the position of the umbo for conve- 
nience of reference and comparison, instead of stating them sepa- 
rately. 
The Cretaceous forms are capable of subdivision into groups of 
species which resemble each other so closely as to seem, at first 
sight, little more than varieties, though each presents, when care- 
fully examined, some definite modification, and characterizes a par- 
ticular horizon in the English Cretaceous series. This peculiarity 
has led me to select as types for description, and as standards of 
comparison, the group from the Folkestone Gault, which not only 
contains the best-known and most perfectly preserved shells, but 
also occurs at about the middle of our Cretaceous series. They 
are, moreover, singularly abundant, bemg represented in my own 
collection alone by so great a number of specimens that the approx- 
imate limits of their variation are easily ascertainable. 
OvaTz. 
OVATE LEVIGATA, 
In this group the shells are smooth or marked concentrically only, 
with scarcely any trace of ribs or pectinate structure. The um- 
bones are situated at about one fourth the entire length of the shell 
from the anterior margin. The lunule is small and usually ill 
defined. The lip is smooth, without notches. The shell is thick, 
and very nacreous internally. 
There are two distinct types in the group, one represented by 
