384 M. Von Buch on the characteristic Fossils 
has not yet been figured, was brought from the Black Hills in 
lat. 46° *, by F. Romer. 
2. Tae TRIGoNIA. 
Not only by their numbers, but still more by the richness of 
their specific divisions, by the peculiar prominence of individuali- 
zation, do the species of the remarkable genus Trigonia attain 
their maximum point in the lower chalk. They do not long 
maintain themselves at this elevation ; like so many other races 
they quickly decline again, and the only species of Trzgonia still 
living, in the warm waters of the Pacific near the Fidji islands, is 
but a very miserable remnant of the fine forms which are still 
preserved in the rocky strata. From this multiplicity of forms 
many might easily be selected as fully characteristic of the chalk ; 
these would however be isolated, and found not universally, but 
perhaps only in a few localities. On the contrary, that remark- 
able family of Trigonie, which Agassiz has named ‘ Trigonie 
scabre,’ is dispersed over every land where the chalk occurs, and 
such a Trigonia is alone sufficient to determine the age of the 
formation in which it occurs. The peculiar and prominent cha- 
racteristic of these forms is a remarkably produced ventral mar- 
gin, when the area of the posterior side is brought, as usually 
happens, into a concave position. This is produced by the very 
oblique angle at which the posterior and anterior sides meet at 
the hinge, an angle which seldom exceeds 60°, whereas in other 
families these two sides meet at a right angle. Along with this 
all the Scubre appear much intumesced at the hinge, and fall 
quickly off with some degree of acuteness before attaining the 
posterior termination. The sides are adorned with ribs, running 
down perpendicularly, of which only a few curve round below 
the hinge and become united on the anterior surface. These ribs 
are divided, crenulated, by more or less acute divisions, placed close 
together, and they are also sometimes slightly curved towards 
the posterior area, when the side of the shell declines somewhat 
more quickly backwards. From such minute distinctions Agassiz 
and D’Orbigny have formed many distinct species, Trzgonia scabra, 
aliformis, carinata and others. However, the characters on which 
these various species have been founded have no fixed limits, but 
pass insensibly into each other ; the numerous crenulations of the 
Trigonia carinata are easily lost on the Tr. scabra and aliformis ; 
the depression of the posterior part of 7’r. aliformis is very often 
altogether wanting. On the other hand, the number of the ribs, 
with nearly equal size, continues almost constant. Eighteen to 
twenty ribs are almost invariably counted on the sides of these T77- 
* A description of this new species is then given by the author.— 
TRANS. 
