140 J. S&S. GARDNER ON BRITISH CRETACEOUS NUCULIDA, 
strie. Itisa little larger, and more compressed, and has more central 
umbones than D’Orbigny’s description admits, and he does not notice 
the fine striz ; but actual comparison shows that those from English 
and French localities belong to the same species. From its likeness 
to the recent ZL. lanceolata, Lamarck, it seems probable that it gaped 
at the extremities, and in that case it would fall into H. and A. 
Adams’s subgenus Adrane. There does not appear to be any repre- 
sentative of this type at Blackdown. 
Dimensions tabulated*. 
Length. Height. Depth. Pesition 
mm, mm. mm. of umbo) eee 
en 10 8 5 a 119° 
Tees ea u 7 6 } 100° 
L. subrecurua, ......-+.- 18 10 10 a } — 
See 2 ee 10 "7 6 } is 
i, Tc iB "7 5 eee } ai 
1: Hae ee oe 7 aes 100 f 145° 
jyet, (RAGES = ah. aN. 6 3s 
if aeerer ee aN 6f. 4s Ug ae } me 
Eis SOMO 8 oe cree: croak 8 4 3 a } 135° 
i. Silea d eaet oete 21 7 wee } 135° 
L. Spaipulaignse scans = 4 18 7 0 He j 135° 
To sum up, it does not appear that the Nuculide will ever be of 
great importance as aids to the field geologist or in stratigraphy. 
Their species seem to have been so slowly modified that the lapse of 
time between one Cretaceous stage and another was insufficient in 
most cases to effect any change that can readily be grasped without 
very close comparison ; and it requires considerable study to convince 
us that the seemingly identical Neocomian, Gault, and Blackdown 
species possess characters by which they can with certainty be 
distinguished. The pectinate forms, however, NV. pectinata and 
JV. bivirgata, by their very easily recognizable characters and their 
* The dimensions are given in millimetres. Average specimens are selected 
as far as possible. The position of the umbo is expressed in the percentage of 
the entire length of the shell at which it is situate from the anterior margin. 
The angles are taken from D’Orbigny, as I possess no instrument capable of 
including any so wide. The measurements in the last two columns are not 
quite reliable. 
