BRACHIOPODA. 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, Dall, 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 IV 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 

 butti'ess 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 {post-7nedian scar, Davidson and King). 



Trimerella is represented in American faunas by five species : T. grandis and 

 T. acuminata, Billings ; T. Billingsi, Dall ; T. Dalli, Davidson and King ; T. 

 Ohioensis, Meek. All- these are known to occur in the Guelph limestone of the 



