38 PALEONTOLOGY 



SUB-ORDER 2. Terebratulacea. Loop-bearing Telo- 

 tremata, with punctate shell, foramen encroaching on 

 the umbo (epithyrid), and deltidial plates not bounding 

 the foramen laterally. Common form, ovoid and smooth ; 

 sometimes with a few coarse plications, consisting of a 

 radial elevation (fold) on one valve, and a corresponding 

 depression (sulcus or sinus) on the other; very rarely with 

 numerous ribs. Ventral umbo and foramen usually much 

 larger than in rhynchonellids. 



The terebratulids first appear at the opening of the 

 Devonian period. One of the Devonian genera has so 

 many peculiarities that it is best described apart from 

 the bulk of the sub-order. 



Stringocephalus buvtini of the Middle Devonian is a 

 large shell, nearly circular but for its straight hinge-line 

 and high and pointed (rostrate) ventral umbo. Unlike 

 other terebratulids it is hypothyrid, and the deltidial 

 plates arch over and meet above the foramen. The shell 

 does not show the typical terebratulid pattern of puncta- 

 tion. The loop is long and wide, parallel to the margin 

 of the dorsal valve ; and there is a median septum in 

 each valve. The cardinal process is so long that it has 

 to bifurcate to avoid the ventral septum. 



The remainder of the. terebratulids are divided into 

 short-looped forms with obsolete dorsal septum and rarely 

 with dental plates, and long-looped forms with strong 

 dorsal septum, and in most cases dental plates. 



The short-looped forms begin with the Devonian 

 Centronella, smooth, with a very simple loop, not 

 doubled back like that of later genera. Dielasma (Dev.- 

 Carb.) has a doubled-back loop, large crural lamellae 

 running towards one another from the top of the 

 descending lamellae, and strong dental plates. Similar 

 forms without dental plates are common in the Jurassic 

 and Cretaceous, and are usually placed in the genus 



