20 
PATAGONIAN EXPEDITIONS ! PALAEONTOLOGY. 
gonia conf. aliformis Park., which may possibly be identical with our spe- 
cies, but the single specimen collected was too imperfect for full description. 
It was associated with an oyster identified with Exogyra couloni Defr. 
Locality and position . — Represented by 13 specimens from the Ammo- 
nite (Belgrano) beds at mouth of canon four miles east of Lake Pueyrrydon 
and by four specimens from the same horizon ten miles east of the lake. 
Trigonia heterosculpta sp. nov. 
PI. IV, Figs. 16-18. 
Shell rather small, ovately trigonal, moderately convex ; anterior end 
broadly rounded, rather prominent ; ventral margin gently convex ; pos- 
terior end slightly produced, narrowly rounded at the extremity and very 
obliquely subtruncate above ; dorsal margin almost straight (very slightly 
concave) from the beak to the posterior end ; beaks not very prominent, 
situated about one third the length of the shell from the anterior end ; area 
and escutcheon narrow, not very sharply defined on the adult shell, both 
being destitute of sculpture other than rather prominent lines of growth 
and a broad furrrow above the middle of the area. The sculpture of the 
young shell, as seen on the beaks and on small specimens, is entirely 
different from that of the adult, and the form also is different. In young 
shells, seven millimeters or less in length, the height and length are about 
equal and the sculpture consists of strong concentric ribs, parallel to the 
growth lines and about as prominent on the area as on the anterior por- 
tion of the shell. At this stage the area is bounded above and below by 
well-marked carinae and the small escutcheon is smooth. As the shell 
grows, the next two or three ribs become swollen and bent downward a 
short distance in front of the area, then one or two form V-shaped angles 
there, and finally in the adult form of sculpture there are two distinct sets 
of costae — one set consisting of slender, smooth costae resembling those 
of the young shell, but ranging obliquely backward and downward from 
the front of the shell until they almost meet the other set, the number 
varying from 6 to 1 2 or more according to age ; the posterior set consist- 
ing of fewer and larger, more nearly vertical, smooth costae that range 
downward from the margin of the area to the ventral border. 
The sculpture is thus seen to agree closely with that of the Undulatae 
section of Trigonia, except that the area is not so well defined. The 
