674 



MOLLUSCOIDEA. 



and it is represented at the present day by a number of living 

 forms. 



In the genus Terebratula itself, as now usually restricted, the 

 shell (rig. 541) is oval or circular, with a smooth surface, and either 

 simply rounded in front, or exhibiting a mesial fold in the ventral 

 valve, to which corresponds a sinus, with a lateral fold on each side, 



in the dorsal valve. The beak of 

 the ventral valve is short, with a 

 wide foramen, below which is a 

 deltidium. The brachial loop (fig. 

 542) is short, and does not extend 

 to more than a third of the distance 

 between the hinge-line and the an- 

 terior margin of the shell. The 

 oldest forms of the Terebratulce ap- 



Fig. 542. — Terebratula sacculus — Car- 

 boniferous. The right-hand figure shows 

 the interior of the dorsal valve with the 

 loop. (After Dawson.) 



pear in the Devonian, and other 



forms are found in the Carbonif- 

 erous and Permian deposits. These 

 early examples of the genus differ from later types in the possession 

 of strong dental supports beneath the hinge of the ventral valve. 

 Throughout the Secondary rocks, and especially in the Jurassic and 

 Cretaceous formations, the genus is largely represented, while there 

 is also a limited number of Tertiary types, and several living species 

 are known. 



In Terebratulina (fig. 543) the exterior of the shell is ornamented with 

 dichotomous radial striae, and the brachial loop is very short, and is con- 

 verted into a simple ring. The species 

 of Terebratulma range from the Jurassic 

 to the present day. 



The genus Waldhcimia is the type of 

 another series of the Terebratulidce, in 

 which the brachial loop is elongated, its 

 length equalling at least half that of the 

 shell itself (fig. 540). In addition to the 

 presence of a long brachial loop, another 

 distinctive character of Waldheimia is 

 the existence of a more or less developed 

 median septum in the interior of the dorsal valve. The species of Wald- 

 heimia appear to commence in the Lias, and the genus is well represented 

 at the present day. There are no dental plates in the ventral valve of 

 Waldheimia, but such plates are developed in the closely allied Zeilleria 

 of the Jurassic, Cretaceous, and older Tertiary formations. 



In the Silurian, Devonian, and Carboniferous genus Centronella (fig. 

 544), the shell has the general characters of Terebratula, but the brachial 

 loop consists of two delicate ribbon-like lamellae, which extend about 

 one-half the length of the shell. " These lamellae at first curve gently 

 outwards, and then approach each other gradually, until at their lower 

 extremities they meet at an acute angle ; then, becoming united, they 

 are reflected backwards towards the beak, in what appears to be a thin, 

 flat, vertical plate. Near their origin each bears upon the ventral side 



Fig. 543. — Terebratulma s?ebstriata 

 Upper Jurassic, of the natural size 

 (After Zittel.) 



