PLACENTICERATID®. 205 
persistent in the type specimen than in other fossils, and this may be a 
specific character, although the condition of other casts does not enable 
me to determine this. 
Locality: Presidio del Norte, Chihuahua, Mexico. 
Age: Probably same as guadalupe. 
PLACENTICERAS SYRTALE (Morton). 
Pl. XXVII, figs. 15-17; Pl. XXVIII, figs. 1-6. 
Ammonites syrtalis Morton, 1834, Synop. Organic Remains, pl. 16. 
Morton’s original specimen is probably a dwarf. At any rate, the 
shell is in its anagerontic substage, and the large nodes given in Morton’s 
figure belong to this age. The diameter is 75 mm., and it is consequently 
smaller than the specimen of var. halei below described; nevertheless the 
gerontic stage has begun, as is shown by the great enlargement of the last 
pair of tubercles and the depression of the venter, and there is no living 
chamber. When this was present, the diameter was probably about the 
same as in the Alabama specimen. The tubercles appear earlier than in 
var. halei and are larger at the same age. 
A specimen from Fort Worth, which shows the typical characters of 
the figure given by Morton, is 97 mm. in diameter. The outer volution is 
42 mm. from line of involution to venter and the opposite is 31 mm. The 
large size of the umbilicus is due to the recession of the outer volution, 
which is in its metagerontic substage, and the shell consequently was almost 
wholly outgrown. It has the large mner nodes, and as these are not so 
numerous as the next outer row of smaller ones there is a distinct aspect 
of bifurcation in the fold-like costz that here and there connect them 
throughout the ephebic and gerontic stages. The venter has a narrow, 
concave zone bordered by elongated tubercles forming a crenulated border 
on either side in the ephebic stage. These are more closely set than in 
var. halei from Alabama. The inner row of nodes, as in P. intercalare, 
does not hold to the line of the umbilical shoulder, but recedes outwardly 
in the gerontic stage, and this stage comes in much earlier than in intercalare 
in all of these specimens. 
The venter has become rounded on the outer quarter of the last 
volution, the ventral line of tubercles being lost. The lateral nodes, 
however, remain prominent, showing that the last or paragerontic substage 
of senile development has not been reached. The outer row is nearer to 
