148 
AMMONITES Smith*. 
TAB. CCCCVI. 
Spec. Char. Depressed, ribed, and keeled ; 
inner whorls few, almost wholly exposed ; 
ribs slightly curved; keel obtuse; sides 
flattened ; aperture oblong. 
A beautiful pearly species that seldom acquires a di- 
ameter of two inches * the flatness of its last whorls, 
distinguish it, independently of size, from others simi- 
larly figured ; in its infancy, (see fig. 1.) it is wholly 
plain and almost globose, with an umbilicus ; as it 
grows, its ribs appear gradually in short risings near the 
umbilicus ; the keel is next formed, and at length the 
whorls assume their flat shape, and the ribs their full ex- 
tent ; figs. 2, 3 and 4. 
Occurs sparingly dispersed through the same stone 
as A. planicostatus* (tab. 73,) and is mentioned at p. 
168 ofVol. I. 
This is named in honor of Mr. W. Smith, the Author 
of a Geological map of England, &c. w 7 hose discovery of 
the regular succession of the strata, and the means of 
distinguishing them by their organized contents, has 
laid the foundation for all our Geological knowledge of 
England. 
I have taken this opportunity of adding a portion of 
the outer whorl of an old shell of A. pjanicostatus, (see 
fig. 6.) to shew that by age it acquires spines upon the 
sides of the flattened portion of each rib, a circumstance 
feat ought to be noticed in the specific character. 
Figs. 5, 6, and 7, are several states of A. planicostatus. 
•So the name should have been spelt. 
