Gl 
TRIGONIA gibbosa. 
TABS. CCXXXV. & CCXXXVI. 
Spec. Cuar. Transversely oblong, gibbose, slightly 
arched, concentrically undulated, or irregu- 
larly sulcated; anterior side separated by a 
broad longitudinal furrow or plane space. 
Var. 8 with large, concentric, interrupted, or 
nodose ridges. 
caer a ened 
A. pecuttar character is given to this 'Trigonia by seve- 
ral deep furrows that mark sudden increases of size; the 
first is placed about three-quarters of an inch from the 
beak; the next as much further; then several more 
approaching nearer each other as they come to the 
front. The breadth is somewhat greater than the length ; 
the posterior side arched; the beaks rather prominent, 
incurved ; the anterior side is nearly plain, divided into 
three parts by obtuse bends, the central one forming a 
straight edge; the back is concave. It seems to have 
been a strong heavy shell, as the cast is very thick. 
In var. 6, the undulations which in var. « are only con- 
spicuous near the beaks are large all the way to the front, 
and much interrupted in parts, so as to form irregular 
blunt knobs: in other respects there does not appear to 
be the slightest difference. 
The casts represented on plate 235, and upper figure 
of plate 236, are silicious: they are from the Limestone 
of Tisbury, in Wiltshire; it is remarkable that the ex- 
ternal cartilage has been cast in Silex, as well as the 
less perishable shell, and is thus well preserved. The 
