BRACHIOPODA. 305 



The actual union of the lateral lamellae of the brachidiura witli the median 

 septum in this shell is the earliest evidence and only known instance in 

 palseozoic faunas of a condition which is prevalent among the terebratuloids of 

 existing seas. The investigations of Davidson, Dall, Friele, CEhlert and 

 Beecher have shown that in Terebratella, Maqasella, Kraussina, Platiuia, 

 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 tlms 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 Terebratellid^ has passed, is 

 found to be very simple, sliowing only the primary completed calcification of 

 the lateral branches or descending lamellag, and aflbrding 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 Terebratellidje. 



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-beaiing 13iachiopO(ln, by Charles E. Bkkchkk ; Transac- 

 tions of the Connecticut Academy, vol. ix, p. 376, pi. i. 1893. 



