CLASS II 



BRACHIOPODA 



405 



Subfamily A. Megathyrinae Dall (emend. Beecher). 



Terehratellidae in tvhich the loop is composed of descending branches only, passing 

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

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

 two following subfamilies. 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. Recent. 



Zellania Moore. Minute shells 

 without a loop, but with a median 

 septum in each valve. Lias ; Europe. 



Argyrotheca Dall (Cistella Gray) 

 (Fig. 605). Externally like 3Iega- 

 thyris, but without lateral septa. 

 Cretaceous to Recent ; Europe. 



Megathyris d'Orb. (Argiope Des- 

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

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

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



Fio. 605. 



Argyrotheca hilo- 

 cularls (Desl.). 

 Cenomanian ; La 



Fi(i. 1506. 



Megathyris decollata (Chem.). Re- 

 cent; Medicerranean. Interior of 

 dorsal valve, ■i/i (after Bavidson). 



long 



and 

 the 



Subfamily B. Dallininae Beecher. 



Terebratellidae 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 Dcdlina. 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- 



FiG. 60S. 



Fig. 607. 



TrlgoneUlna pectuncula (Scliloth.). Upper 

 Jura ; Engelhardsberg, Frauconla. A-C, 

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

 of loop, enlarged. 



Mnehlfpldtlxi truncata (Gemm.). . , , . 



Recent ; Mediterranean. Interior briate descending 



of dorsal valve, c, Crura ; (7, Dental i \i aa a -n c\ 



sockets ; «, Jugum ; /, ?, Ascending Draucnes anu 



and descendiiig branches of loop ; simple aSCeudiug 

 p. Process attached to median sep- ^ <-' 



turn ; pr, Cardinal process ; S, branches. Jura. 

 Median septum. 



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

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



