THE BRACHIOPODA 39 



Terebratula, but that genus in the strict sense is Cainozoic 

 only. A few well-marked Mesozoic forms have been 

 given separate generic names. Thus Terebratulina has a 

 ring-like (annular) loop, and is finely ribbed ; Glossothyris 

 has a very small loop and an anterior dorsal sulcus ; 

 Epithyris (Fig. 5, a) has a small dorsal septum, a rather 

 large loop, a rather unusual beak, and more or less of a 

 quadriplicate margin ; Pygope and some other genera 

 pass through a Glossothyris -like stage, and the lateral out- 

 growths meet and unite again around a central perfora- 

 tion (these are found in the transition beds from Jurassic 

 to Cretaceous in the Alpine region) ; Liothyrina (Cret.- 

 Rec.) has a number of radial grooves in the inner surface 

 of the ventral valve (well seen in internal casts). 



Although long-looped forms are known at least from 

 the Trias, it is in the Lias that the simplest form 

 first appears. This is the minute Zellania, in which the 

 loop is incomplete and the dorsal septum anterior in 

 position. This is followed by Megathyris, a broad form 

 with a loop not unlike that of Stringocephalus, but sup- 

 ported by two lateral septa as well as the median 

 septum. Several allied genera are known in the sea to- 

 day, survivors of this primitive group. The higher 

 recent genera fall into two families, one in northern, the 

 other in southern seas, and the highest in each case 

 (Dallina, northern; Magellania, southern) has a very 

 similar adult loop, but they attain it respectively through 

 two quite distinct series of metamorphoses, and the 

 temporary loop-forms of these genera correspond to adult 

 loop-forms of other genera, mostly extinct. These facts 

 are illustrated in Fig. 9. It will be noticed that some 

 of the intermediate stages are represented by recent, 

 not fossil, genera. This can be accounted for by the 

 imperfection of the geological record, for the genera 

 in question are small forms, not easily preserved as 



