BRACHIOFODA. 
305 
The actual union of the lateral lamellse of the brachidium with the median 
septum in this shell is the earliest evidence and only known instance in 
palaeozoic faunas of a condition which is prevalent among the terebratuloids of 
existing seas. The investigations of Davidson, Dall, Friele, OEhlert and 
Beecher have shown that in Terebratella, Magasella, Kraussina, Platidia, 
Bouchardia, and, indeed, all genera where the median septum is highly devel¬ 
oped, the calcification of the lamellae of the brachidium begins quite as soon 
from the lateral walls of the septum as from the crural bases on the hinge-plate. 
Calcification thus proceeds both posteriorly and anteriorly. In all modifica¬ 
tions of the brachidium attendant upon the resorption of later growth, the 
median septum is most intimately concerned, and in the terminal stage of such 
modifications every trace of this septum may have been removed (compare 
Magellania venosa, Macandrevia cranium). 
The mature condition of Tropidoleptus, when compared with the variations 
from resorption, through which the loop of the Tehebratellidm has passed, is 
found to be very simple, showing only the primary completed calcification of 
the lateral branches or descending lamellae, and affording no evidence whatever 
of any modification resulting from resorption of the calcified tissues. Its con¬ 
dition is directly comparable to the mature form of the loop in Platidia, and to 
what is termed by Beecher the platidiform stage in Muhlfeldtia and Macandre¬ 
via. The transverse and strongly plicated valves with well developed cardinal 
areas, are features in harmony with the condition of the brachidium, as similar 
characters are borne in the primitive conditions expressed by the mature Kraus¬ 
sina, Cistella, Megathyris, etc. Immature specimens of Tropidoleptus fre¬ 
quently show an uncompleted condition of the calcification of the brachidium. 
All the evidence thus points to the conclusion that this interesting genus is an 
early representative of the family Teeebratellid^. 
The wide distribution of T. carinatus through the Devonian of North and 
South America has already been referred to in the discussion of the genus Vitu- 
LiNA. In the argillaceous shales of the Hamilton group in western New York 
* See Revision of the Families of the Loop-bearing Brachiopoda, by Charles E. Beecher; Transac¬ 
tions of the Connecticut Academy, vol. ix, p. 376, pi. i. 1893. 
