I 



BRACHIOPODA 

 Subfamily A. Megathyrinae Dali (emend. Beecher). 



405 



Terehratellidae in luhich the loop is composed of descending hranches only, passing 

 in the highest genus through stages correlative with Gwynia, Argyrotheca and 

 Megathyris. The lower genera do not complete the series, The original stock for the 

 two following suhf amilies. Jurassic to Recent. 



Gwynia King. Minute, elongate-oval, smooth shells with a short, nearly 

 straight hinge-line. Neither septa 

 nor loop. Brachia primitive, consist- 

 ing of a circlet of cirri. Eecent. 



Zellania Moore. Minute shells 

 without a loop, but with a median 

 septum in each valve. Lias ; Europe. 



Argyrotheca Dali (Cistella Gray) 

 (Fig. 605). Externally like Mega- 

 thyris, but without lateral septa. 

 Cretaceous to Recent ; Europe. 



Megathyris d'Orb. {Argiope Des- 

 long.) (Fig. 606). Transversely elongate, plicate shells with long and 

 straight hinge-line. Dorsal valve with three or five septa, causing the 

 brachidium to have four lobes. Jura to Recent ; Europe. 



Fio. 605. 



Argyrotheca bilo- 



cularis (Desl.). 



Cfinomanian ; La 



Manche, i/i. 



Megathyris (lecollata (Chem.). 

 Cent; Mediterranean. Interior of 

 dorsal valve. 4/j (after Davidson). 



Subfamily B. Dallininae Beecher. 



Terehratellidae with the loop composed of descending and ascending lamellae, 

 passing in the highest genera through metamorphoses comparable to the adult structure 

 of Platidia, Ismenia, Muehlfeldtia, Terebratalia, and Dallina. The lower genera^ 

 therefore, do not progress to the final stages. Jura to Recent. Recent genera 

 restricted to boreal seas. 



Platidia Costa {Morrisia Davidson). Small, smooth, biconvex shells with 

 a large pedicle opening common to both valves. In the dorsal valve there 



is a high vertical 

 plate to which are 

 attached the de- 

 scending branches; 

 ascending branches 

 not represented. 

 Recent. 



Ismenia King. 

 Coarsely plicate 

 Shells with the loop 

 consisting of fim- 

 briate descending 

 branches and 

 simple ascending 

 branches. Jura. 



Fin. 607. 



Trigonellma pectuncula (Schloth.)- Upper 

 Jura ; Engelhardsberg, Franconia. A-C\ 

 Shell of the natural size. D, E, Two.views 

 of loop, enlarged. 



Fio. 608. 



Mvehlfeldtia truncata (Gemni.). 

 Recent ; Mediterranean. Interior 

 of dorsal valve. c, Crura;(Z, Dental 

 socketa ; e, Jugum ; /, l, Ascending 

 and descending branches of loop ; 

 p, Process attached to median sep- 

 tum ; pr, Cardinal process ; S, 

 Median septum. 



Trigonellina Buckman (Fig. 607). Jurassic ; Europe. 

 Muehlfeldtia Bayle (Megerlea King) (Fig. 608). Jura to Recent. 



