HUGHES : INGLEBOROUGH. 
299 
expands chiefly along the hinge hne, so that it is cigar- shaped, 
but later on it ceases to extend so rapidly in the direction of its 
breadth, but develops the great wavy curtain-like shell which 
is so characteristic of the species. The figure on Plate XXXVII. 
shows an adult form and a young specimen on the same piece 
from the top of the Upper Scar Limestone on Simon Fell. The 
figure on Plate XXXVIII. represents a full grown specimen 
from the top of the Great Scar Limestone at Hull Pot — and 
a young specimen from the same locality is given on Plate 
XXXVI. , Fig. 3. Sometimes the shell is developed earlier in 
the direction of its length, and in that case a more rotund 
form is produced, as, for example, that shown on Plate 
XXXIX., Fig. 1. 
The specimens found at both of these horizons show great 
variation in the amount of corrugation of the shell. To keep 
up the figure, in some the curtain is more pulled out and in 
others more squeezed up into plaits, and the folds are in some 
very irregular, while in others they are stronger and more sym- 
metrical, as in the specimen figured Plate XXXIX., Fig. 2. 
In the Black Marble, above the black limestones which 
form the top of the Great Scar, there is another form, which in 
its younger stages may be confounded with the younger forms 
of P. gigantea. This is the P. latissima of Phillips (see Plate 
XXXVI., Figs. 1 and 2). Where only a fragment of the shell 
is seen, especially when that happens to be a more rotund form, 
it may be mistaken for a variety of P. costata (see Plate 
XXXIX., Fig. 3), from the even ribbing and obscure spine 
scars, but in good specimens the difference of form and the 
corrugations on the wings of P. costata enable us to distinguish 
them easily. 
The examination of a large number of examples generally 
enables us to arrive at a pretty satisfactory conclusion as to 
their affinities, but we want scores, perhaps hundreds, of speci- 
mens to test the range of individual variation, v/hich in the case 
of the species we are considering is very great, and these 
difficulties explain and justify the remarks on pp. 254-256 as to 
the work among the fossils of Ingleborough that offers itself to 
the student. 
