BRA CHIOPODA. 
35 
out the Trimerellas, the scars are much less sharply defined than in the other 
genera of the family. Thus, there may be no sufficient reason to doubt that 
when this apophysis is developed, it is analogous to the cardinal process in the 
articulates; and we have confirmatory evidence of this function of the process 
in its great development in Lakhmina. 
The platforms in the Trimerellas have usually about the same relative degree 
of development, being generally slightly larger in the brachial than in the 
pedicle-valve, and vaulted for almost if not quite their entire length, except in 
the species T. Billingsi, Dali, where they are solid for about two-thirds their 
length. This species again indicates the tendency toward Monomerella in the 
gradual obliteration of these platform-vaults. The usual faintness of all the 
muscular impressions must be due, in an important degree, to the condition of 
preservation of the fossils. The shells were among the most ponderous of all 
brachiopods, and the muscles to wield them must have been of great size and have 
produced strong scars. The American specimens of Trimerella are almost 
invariably internal casts in dolomitic limestones, and the removal of the shell- 
substance left great space for the operation of crystallizing forces which would 
go far toward the obliteration of these impressions. An interesting feature is 
the not infrequent irregular development in the vaults of the platform, one 
being sometimes shorter or broader than the other. A tendency to lateral 
asymmetry in the growth of the umbonal portions of the shell is sometimes 
seen, resulting in a deflection of the beak and platform to one side. (See 
Plate IY a, fig, 3.) 
In some species, noticeably in T. acuminata, Billings, the platform of the 
pedicle-valve bears a deep median furrow, beginning at the base of the cardinal 
buttress and narrowing anteriorly. This feature is developed, to a greater or 
less degree, in most of the species, and may represent the position of the ovar¬ 
ies {jpost-median scar, Davidson and King). 
Trimerella is represented in American faunas by five species: T. grandis and 
T. acuminata, Billings; T. Billingsi, Dali; T. Dalli, Davidson and King; T. 
Ohioensis, Meek. All these are known to occur in the Guelph limestone of the 
