390 SPENCER : THE YOREDALE AND MILLSTONE GRIT ROCKS. 
than occurs in the Yoredale beds. In the shales of the Third Grits 
passed through in sinking the shafts on Wadsworth Moor for 
the Hahfax Waterworks, beds of black fossiliferous shales full of 
Aviculopecten, Goniatites, &c., 10 feet to 20 feet in thickness, 
were met with. 
The marine shells and fish-remains found in the roof of the 
Halifax Hard Bed Coal also belong to the same genera as those 
of the Yoredale shale, with pei-haps a few additional forms. 
The gradual development of Goniatites listeri is well illustrated 
in these beds. In the Yoredales this shell is of small form, and 
good specimens are somewhat rare. They generally occur in the 
shales of the Yoredale, Millstone Grit, and Coal strata in a crushed 
condition. In the Yoredale shales of Todmorden small limestone 
nodules occur in which small specimens of this species are found in a 
good state of preservation ; all the specimens I have seen are of small 
size, but as we trace their crushed forms through the shales of the 
Millstone Grit they seem to gradually increase, both in numbers 
and in size. In the marine bed over the Halifax Hard Bed Coal 
Goniatites listeri occurs in all sizes from that of a pin's head, or 
even less, to others from four to six inches in diameter, and in vast 
numbers. Only a few other species of Goniatites occur in the 
bed, and none of them seem to have undergone any change in 
form. The beautiful little shells, G. looneyi and G. gilhertsoni 
are of the same size as they are in the Yoredale beds and in 
about the same relative proportion. Goniatites reticulatus, the 
most common form in the Yoredale rocks, appears to have 
degenerated in its journey through the Millstone Grits. This 
beautiful fossil assumed several distinct forms from its birth 
to its adult stage, when it reached a diameter of three or 
four inches. But it gradually dwindled away both in size and 
in numbers in its j^assage through the grit rocks, and seems to 
have died out at the end of the Third Grits and the overlying 
shales. 
