BRACHIOPODA. 
7 
faunas appearing on the earth, its persistence in time is nevertheless unequaled 
by that of any other known genus of organisms. 
The palaeozoic Lingulas, sensu stricto, have not shown variations of sufficient 
importance to permit a thoroughly satisfactory subdivision of the genus. As 
the interiors and muscular scars of different species become known, they re¬ 
veal a closer alliance with one another, and with the recent type of the genus, 
than had been suspected. 
There is a growing tendency among authors to remove certain species from 
this genus to Lingulella on account of an apparent inequality in the valves of 
the shell, accompanied by a cardinal area on the pedicle-valve, which seems 
to be grooved or depressed from the apex forward. Great care is needed in the 
accurate determination of these features. The type species, Lingula anatina , 
has not only gaping and slightly unequal valves, but the pedicle-valve has a 
distinct cardinal shelf or area, divided longitudinally by a depression widening 
from the apex anteriorly, and termed by King* the “ deltidium.” This, accord¬ 
ing to Professor King, is usually not well developed, and in the average speci¬ 
mens of L. anatina found in museum collections, ap¬ 
pears to be partially or quite wanting, from accidental 
causes. The deltidium is bounded at the sides by 
elevated ridges, which, at the anterior ends, are each 
developed into a small callosity. “ The ridge-callosi- Fig . 2 . Ungula anatina . cavdinal area 
-i i i ,i of pedicle-valve, after King, a, del- 
ties are no doubt insignificant; nevertheless they tkiium;&,deitidiairidges;AtWai 
j • , r> • , , , , callosities; d, areal borders. 
appear to be the rudiments ol important structures. 
Apparently they have become so far developed in Lingula Lesueuri, as to serve 
to articulate the valves. If I am correct.this species cannot 
belong to the genus in which it has been placed.” f In the opposite or brachial 
valve is a slight area without a deltidium, but bearing concentric growth-lines, 
and a very faint longitudinal groove. 
It is to be expected that we shall find similar features more or less devel¬ 
oped among the fossil Lingulas. The broad, depressed deltidium of L. anatina 
* Annals and Magazine of Natural History, July, 1873. 
t King, loc. cit., p. 13. 
