278 SYNOPTIC COLLECTION. 



fig. 3, side view), in which a primitive loop (fig. 4; fig. 5, 

 side view of same) descends from the brachial valve. 

 This is made by two leaf-like parts or lamellae which are 

 attached to the crura and which unite in the median line 

 to form the broad loop. 



The hinge area with its ridges and the muscular scars 

 are seen in fig. 6. 



Stringocephalns burtini Defr. (PL 682 ; No. 683) is a 

 Devonian representative of a more specialized family. 

 In the young shell (PI. 682, fig. i) the triangular pedicle 

 passage or delthyrium is open or sometimes partly closed 

 by partially developed deltaria ; in the adult (PL 682, figs. 

 2-4 ; No. 683) it is wholly closed excepting the pedicle 

 opening (fig. 2), while in the old age stage (fig. 5 ; No. 

 683, specimen on the right) the coalescence of the plates 

 into one plate (called the deltarium and the pseudodel- 

 tidium) is almost complete, and the pedicle opening is 

 greatly reduced in size. 



The arm supports consist of a long loop which extends 

 around the margin of the brachial valve. Its form and 

 the radial filaments extending from it are seen on the left 

 of PL 682, fig. 3. The strong cardinal process extending 

 from the brachial valve towards the large median septum 

 of the pedicle valve on the right is also well seen in fig. 3. 



The changes which a Centronella-like loop often under- 

 goes are well illustrated in the Palaeozoic Brachiopod, 

 Dielasma turgidum (PL 684, figs. 1-5). Fig. i is the 

 Centronella-like loop of the young. The pointed ante- 

 rior portion is resorbed and two points extend forward 

 (fig. 2). The process of resorption continues till the 

 loop has the form of fig. 3. In the loop of the adult the 

 two branches or lamellae diverge, as seen in fig. 4, x 6; 

 fig. 5 is a side view showing the crus and loop on the 

 right and the median septum on the left. 



Among the instructive forms of the Telotremata living 

 to-day are Terebratula and Terebratulina. 



One of the forms of Terebratula (T. insignis d'Orb.; 



