56 MALONE JURASSIC FORMATION OF TEXAS. [bull. 266. 
TRIG^OlSrilD^E. 
Genus TRIGONIA Brug. 
Trigonia vyschetzkii Cragin. 
PI. VIII, figs 1, 2; PI. IX, figs. 1-3. 
Trigonia vyschetzkii Cragin. ISO.",, Fourth Ann. Kept. Geol. Survey Tex., pt. 2. 
p. 215; and. L897, Jour. Geol., vol. f>. No. 8, pp. 816, SIT. 
Cf. Trigonia solognreni Felix, 1891, Beitr. Geol. u. Fal. Mex., pt. 3, p. 17!>, 
PI. XXVII. figs. 2. 2a. 
Shell large, of moderate convexity, transversely subquadrate; pos- 
tero-dorsal side straight; anterior and posterior sides truncate; base 
gently curved; beaks only moderately arched; area flattened, occuj 
pying about one-fourth of the valve, bordered above and below by 
tuberculated ridge or angulation, and traversed by a median groove 
which is accompanied by a third such ridge less developed than those 
of the border, a large distal pari of it obliquely erossed in most 
instances by numerous strong, more or less irregular and interrupte< 
folds, or ornamented on the proximal part with rows of tubercles 
by whose increasing confluence on the intermediate and distal part] 
the folds are produced, the tubercular endings of these folds usuall; 
descending across the entire outer slope of the bounding ridge, and 
in some cases even passing a little beyond it; escutcheon depressed, 
crossed with a series of transverse, somewhal oblique, straight, or 
in pari slightly curved, tubercular costellse; preareal surface orna- 
mented with about IT) gently curved nodose costae, which are sub] 
vertical near the infraareal ridge and make with the latter at first 
an acute and then approximately a right angle, descending for the 
most part obliquely, though steeply, to the base: the costse attenuated 
and simpler near the area, becoming resolved into irregularly more 
and more robusi nodes a- they recede from it. the posterior and inter- 
mediate costae thus enlarging throughout, while a few of the shortei 
anterior ones, turning forward at their lower ends across the flat- 
tened front of the shell, are here again reduced to nearly simple 
ridges which dwindle and disappear before reaching the valve mar- 
gin; nodes mostly close ranked, commonly 12 to 16 on the' larger ribs 
of adult shells, rounded, often more or less compressed so as to tre'ne 
with the growth lines, a rib ofton having a single very large node 
near the' basal margin of the' valve' especially thus elongated; inteq 
vals between the costse coarsely concent rically striate'. The separatioi 
e>f the siphonal currents is indicated, as in the Clavellatae and Scabrsa 
by a rielge on the interior of the shell. The ornamentation shows conj 
siderable mutability, and a variety occurs in which the' entire are: 
is covered with small compressed tubercles. In one specimen the 
eostelbe of the escutche'on are simple. 
