BRACHIOPODA. 
49 
senting the last place of attachment. In L. anatina these median scars are 
usually localized and excavated on the margin 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 the shell, the natural result of the union of these t 
depositions is a median septum * 
Among the Lingulas which have passed under our 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 muscular 
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 palaeozoic 
faunas, of some linguloid shell which will show 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 application as to include all such septal 
phenomena in the brachiopods. It does apply to Lingula anatina, and to all Lingulas, as far as we are 
aware, whether the septa be axial or lateral; but it would be difficult to thus explain the origin of the great 
anterior extension of the septum into the brachiocoele, as seen in Lingulasma, Trimerella, etc. Such great 
vertical plates may, however, have had their source in septa of the character of those in Lingula. Among 
the articulate brachiopods, the origin of median septa from similar causes is often apparent. 
