167 
AMMONITES planicosta. 
TAB LXXIII. 
Spec. Char. Depressed,, volutions six or more, not 
concealed; (diverging costas) or radii numerous, 
obtuse, flatted at the front; mouth circular, 
slightly indentated at the back by the preceding 
whorl. 
Between the rising stria: of this shell are rounded furrows; 
the costae are prominent on the sides and straight, as they 
go round the front they widen, are flatted, and incline 
towards the mouth. The siphunculus is only now and then 
preserved so as to be discernible. 
I have long desired to acknowledge the kindness of 
John Rogers, Esq. of Yarlington in favouring me with 
two beautiful drawings of this fossil, commonly known by 
the name of Marston Stone. It is found at Marston magna 
near Ilchester; at Everskot in Somersetshire; in digging a 
well in Lord Digby’s park at Sherborne; and also near 
Yeovil in Somersetshire in moderate masses, occasionally 
big enough to form tolerable sized sideboards ; Dr. Lettsom 
has a very fine one. It is formed of a dark marly Lime- 
stone, including shells laying in great disorder, and much 
variety is produced by the sections when cut and polished* 
the white shelly remains passing into a buff colour, and 
sometimes finely iridescent both inside and out. The di- 
visions of the chambers lined or filled up with fine brown 
