384 M. Von Buch on the chay^aderistic Fossils 



has not yet been figured, was brought from the Black Hills iu 

 lat. 46°*, by F. Romer. 



2. The Trigoni^. 



Not only by then' 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 Trigonia 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 Trigoniaj which Agassiz has named ^ Trigonia 

 scabrcB,' 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 Scabrae 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 

 andD'Orbigny have formed many distinct species, Trigonia 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 TV. scahra and aliformis ; 

 the depression of the posterior part of Tr. 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 Tri- 



* A description of this new species is then given by the author. — 

 Trans. 



