61 
TRIGONIA gibbosa. 
TABS. CCXXXV. & CCXXXVL 
Spec. Char. Transversely oblong, gibbose, slightly 
arched, concentrically undulated, or irregu- 
larly sulcated ; anterior side separated by a 
broad longitudinal furrow or plane space. 
Var. /3 with large, concentric, interrupted, or 
nodose ridges. 
A peculiar 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. the undulations which in var. a 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 
