148 GEOLOGICAL SURVEY OF THE TERRITORIES. 
straight or very slightly convex, forming, with the dorsal margin, a dis-. 
tinct, slightly obtuse angle; dorsal margin concave, the appearance of 
convexity being increased by the elevation of the umbones; umbonal 
ridge distinct and prominent below, but becoming obsolete towards the 
beak. Surface in front of the umbonal ridge marked by ten or twelve 
curved varices, which start from near the umbonal ridge and extend to 
the front and basal margins, the concavity of their curve being upward 
and forward. The first six or seven of these varices become broken up 
into distinct nodules a short space in front of the ridge. ‘he forward 
ones of each varix or transverse row of nodules are larger than the 
others, and are ranged in a vertical row of strong nodules, which form 
a conspicuous feature of the shell. The corselet, or space behind and 
above the umbonal ridge, is marked only by numerous distinct, nearly 
vertical, lines of growth. 
Length, 44 millimeters; height from base to beak, 35 millimeters; 
height of posterior portion, 20 millimeters. 
This species resembles 7. signata Agassiz, from the lower Jura of Eu- 
rope, but it differs in the character of the noduliferous varices, in hav- 
ing a vertical row of large nodules, in the character of the umbonal 
ridge, and in other particulars. In the possession of the vertical row of 
nodules it resembles T. navis Lamarck, from the Lias of Alsace, but it 
differs from that species too much in other particulars to need compari- 
son. 
Position and locality.—Jurassic strata near the Lower Cafion of the 
Yellowstone, Montana, where it was collected by Dr. A. C. Peale. 
TRIGONIA AMERICANA Meek. 
Plate 38, figs. 1a and b. 
Trigonia americana Meek, 1873, An. Rep. U. 8. Geol. Sur. Terr. for 1872, p. 472. 
Shell subtetrahedral in marginal outline, gibbous in front of the um- 
bonal ridge and compressed behind it; the margin from the beak all the 
way to the postero-basal angle forming a continuous and almost regular 
curve; posterior margin nearly straight, truncating the shell downward 
and backward; dorsal margin gently concave; beaks somewhat elevated 
and having the appearance of being directed slightly backward; um- 
bonal ridge forming a somewhat prominent crenulated carina, which 
‘very gradually increases in size towards and ending at the postero-basal 
angle. Surface in front of the umbonal ridge marked by 15 to 18 sharply- 
raised concentric varices, which become suddenly obsolete just before 
reaching the umbonal ridge. The surface belind that ridge is marked 
by numerous raised, radiating, finely crenulated lines, which are nearly 
uniform in size; but the spaces separating the five or six lower ones are 
a little wider than the others. 
Length from front to posterior margin, 34 millimeters ; height from 
- base to beaks, 31 millimeters. 
This species is of the type of J. costata Lamarck, from the Lower 
Oolite of Europe, but it differs conspicuously from that and nearly re- 
lated species by the uniform character of the radiating markings of the 
corselet. 
Position and locality. Jurassic strata near the Lower Cajfion of Yellow- 
stone River, Montana, where it was collected by Dr. A. C. Peale, who 
found it associated with ZT. montanaensis, Gervillia moitanaensis, and 
Myacites subcompressus. 
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