34 PALEONTOLOGY OF NEW YORK. 



Crescent as in the other valve. Phitform 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- 

 KRELLA and DixoBOLUS. 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, Dall. Messrs. Lindstr()M, 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 

 liiid 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 spt!cies, T. LincUtrwmi, Dull, jiossessys a peculiar umbonal character. The small lateral 

 umbonal chamliers are forced wide ajiarl by the development of a me<Uan, undivided cavity, which makes 

 the umbo hollow for its entii-e length. This cavity is i-epresented in the figures given by Lind.'stkom, David- 

 son and Kisf;, 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 

 \)uttross usually occupies this median position. Several specimens of the pedicle-valve of this species, after 

 h.aviiig the matrix carefully removed by me.ans of caustic potash, give no evidence whatever of this 

 opening, the shell being perfectly continuous from the platform to the deltidium, and we are le>l to suggest 

 that the external opening of this cavity, so out of harmfmy with the typical char.acters of the genus, may 

 be ilue to fortuitous causes. The canlinal buttress, however, is replaced by a remarkably broad and deep 

 excavation. 



