121 
AMMONITES Birchi. 
TAB. CCLXVIL 
Sp. Char. Discoid,, inner whorles exposed ; sides 
concave, armed with two involuted rows of 
spiniform tubercles; front rounded, trans- 
versely and obscurely sulcated ; volutions 6. 
tubercles very numerous. 
About 30 pairs of tubercles decorate each wliorle, each 
pair is connected together by a slight elevation and the 
outer tubercle is the largest. The sulci upon the front 
sometimes continue over the sides but less distinctly. It 
is an essential character that the whorles do not encrease 
rapidly in size, consequently in a large shell they are 
rather numerous and its sides are not very hollow. The 
aperture is transverse, but very little longer than wide. 
This Ammonite bears a strong resemblance to another 
found plentifully at Havre de Grace, in a similar clay 
soil but probably belonging to a different stratum, it is 
highly essential for Geologists to be acquainted with the 
characters that distinguish allied species especially when 
they belong to different formations. In the present in- 
stance the number of tubercles, as well as whorles, may 
be depended upon as sufficiently striding to prevent mis- 
take, the French shell having only half the number of the 
former, and not more than 4 of the latter ; the tubercles 
are also larger and front undulated. When several 
species are nearly alike a descriptive name cannot always 
