BRACHIOPODA. 49 



senting the last place of attachment. In L. anatina these median scars are 

 usually localized and excavated on the mariiin of the shelly thickening which 

 covers the visceral area, and it appears that this thickening of the shell is 

 largely due to the progressive deposition of testaceous matter about the inser- 

 tion of the muscles; where the members of a muscular pair come into juxta- 

 position along the axis of tlie shell, the natural result of the union of these 

 depositions is a median septum.* 



Among the Lingulas which have passed under oui- observation, whether of 

 fossil or recent species, it is seldom that any tendency is shown by these scars 

 to become otherwise than thus terminal and excavated. 



The first deviation in this respect toward the formation of a platform is 

 found, without any known transitional forms, in Lingulops itself; not in the 

 brachial valve, which actually possesses this organ in an incipient stage, but in 

 the pedicle-valve as elsewhere described and illustrated. Here the muscular 

 scars, which indicate large organs for such small shells, are concentrated well 

 toward the middle of the valve, and their anterior edges are distinctly and 

 abruptly elevated, placing the scars, by virtue of the stronger deposition of the 

 shelly matter about the anterior edge of the area of insertion, upon, instead of 

 beneath, the median thickening of the shell. How much of this change may 

 be due to the increase in size and in the work required of the nmscular 

 apparatus does not appear in the minute and fragile shells constituting 

 this genus, except that the impressions are large and all very sharply de- 

 fined (a most striking feature in nearly all of the genera of platform-bearing 

 shells is their great size and weight, necessitating powerful muscles to work 

 them). In view of the close relations of the characters of this valve to those 

 of LiNGULA, we may venture to predict the discovery in the earlier paleozoic 

 faunas, of some linguloid shell which will sliow in both valves just the 

 stage of deviation from Lingula toward Trimerella that is indicated 



* It is not the intention to give this suggestion so broad an aiii>li(-ation as to include all such septal 

 phenomena in the brachiopods. It does apply to Linr/ula anatina, and to all Lingulas, as far !is we are 

 awaie. whether iXv se|ita be axial or lateral ; but it would be difficult to thus explain the oi-igin of the great 

 anterior extension of the septum into the biachiocccle, as seen in Linghlasma, Trimerbll.\, etc. Such great 

 vertical plates may. however, have had their source in septa of the character of those in Lingdla. Among 

 the articulate brachiopods, the origin of median septa from similar causes is often apparent. 



