34 
PALAEONTOLOGY OF NEW YORK. 
Crescent as in the other valve. Platform higher than in the pedicle-valve, 
its vaults extending scarcely so far backward. Anterior median septum long, 
often reaching almost to the margin. Muscular and pallial impressions faint. 
Type, Trimerella grandis, Billings. 
Observations. The species of this genus are subject to some interesting varia¬ 
tions, though, in the development of certain features within the limitations of 
a given species, such great differences are not as noticeable here as in Monom- 
erella and Dinobolus. The relative size of the great cardinal area appears to be 
a permanent character. A tendency toward the subdivision of the umbonal 
cavity by a vertical septum (cardinal buttress) is apparent in all species, and 
constitutes an approach to the large umbonal chambers in Monomerella. The 
great development of the platforms necessitates the abbreviation of the umbonal 
chambers, while, accompanying the obsolescence of the platforms in Monomer¬ 
ella, is a resultant increase in the size of these chambers* In the brachial 
valve the umbo is incurved, and just beneath, or within it, is sometimes devel¬ 
oped a strong process, grooved on its outer surface. This feature is seen in T. 
Ohioensis, Meek and T. Lindstmmi, Dali. Messrs. Lindstrom, Davidson and 
King, and Dall have been disposed to regard this as a cardinal process, fitted 
like a tooth into an excavation at the base of the deltidium of the opposite 
valve. The evidence of such a process affording attachment for muscles open¬ 
ing the valves is generally wanting among the other genera, and other species 
of this genus; and if this process served such a function, we should expect to 
find correspondingly large and deep muscular scars upon the platform of the 
pedicle-valve. Our evidence on this point is rather negative, though through- 
* The Gotland species, T. Lindstroemi, Dall, possesses a peculiar umbonal character. The small lateral 
umbonal chambers are forced wide apart by the development of a median, undivided cavity, which makes 
the umbo hollow for its entire length. This cavity is represented in the figures given by Lindstrom, David¬ 
son and' King, as opening at the hinge-line or posterior margin of the platform. No similar structure, or 
any tendency to its formation, has been observed in any other species ; on the contrary, the strong cardinal 
buttress usually occupies this median position. Several specimens of the pedicle-valve of this species, after 
having the matrix carefully removed by means of caustic potash, give no evidence whatever of this 
opening, the shell being perfectly continuous from the platform to the deltidium, and we are led to suggest 
that the external opening of this cavity, so out of harmony with the typical characters of the genus, may 
be due to fortuitous causes. The cardinal buttress, however, is replaced by a remarkably broad and deep 
excavation. 
