172 Mr. T. H. Withers on some 



aud G peduncular plates (three figured, PI. VIII. figs. 1-3), 

 together with a scutum (PI. VII. fig. 16), were found em- 

 bedded in a small piece of clay measuring barely 1 cubic 

 inch, and may therefore belong to the same individual. 

 This might also be the case with the rostrum (PI. VII. 

 fig. 15) and 8 peduncular plates, which were found together 

 in a similar piece of clay. The remaining single peduncular 

 plate was found together with a carina. 



It is a very significant fact that these peduncular plates 

 were found on three different occasions, and in association 

 with the other valves of P. rigidus, for if there had been a 

 lower series of valves to the capitulum of this species it is 

 extremely unlikely that one would find 15 peduncular plates 

 and not a single example of a valve of a lower whorl. We 

 are therefore led to the conclusion that there was no lower 

 whorl, and that the capitulum of P. rigidus was formed of a 

 single whorl of 8 valves. 



Measurements. The valves here figured from the Gault of 

 Folkestone measure respectively : — 



Length. Breadlh. 



mm. mm. 



Eostrum 8-9 5-7 



Scutum 13-0 5-8 



Upper latus 5'8 27 



Tergum IM 6-7 



Carina 146 6-G 



Peduncular plate (PI. VIII. fig. 1) . . lo 24 



A further peduncular plate has a length of 18 mm. and 

 a breadth of 3'2 mm. Much larger valves are known from 

 the Gault of Folkestone than any of the above, and in the 

 British Museum are four valves with measurements : — 



Length. Breadth, 



mm. mm. 



Rostrum. I. 13631 15-0 8-8 



Scutum. I. 13488 15-2 7-8 



Tergum. I. 13486 19-4 10-6 



Carina. I. 13639 19-3 7-1 



In the Museum of Practical Geology (no. 26854) there is 

 a scutum measuring 18*4 mm. in length and 7*5 mm. in 

 breadth, and this is the largest scutum seen by me. 



The valves from the Chalk Marl of Cambridge are rather 

 small, the largest, a scutum (PI. VIII. fig. 4), being 4*8 mm. 

 lone: and 2'1 mm. broad. 



