442 



MOLLUSCA. 



finally disappear. The Lingulidce commence in the Cambrian 

 period, and have survived to the present day. The Rhyncho- 

 nellidce, Craniadce, and Discinidce commence in the Silurian 

 period, and are represented by living forms in existing seas. 

 The Thecidiidce extend from the Trias to the present day; 

 and the Terebratulidce appear to commence in the Upper 

 Silurian, and are well represented by living forms. In the 



following are given the leading 

 characters and more important 

 forms of the families of the 

 Brachiopoda : 



FAM. I. TEREBRATULID.E. - 

 Shell minutely punctate : ven- 

 tral valve with a prominent 

 beak, perforated by a foramen 

 for the emission of a muscular 

 peduncle, whereby the animal 

 is fixed to some submarine object. Foramen partially sur- 

 rounded by a deltidium of one or two pieces. Arms entirely 

 or partially supported by calcified processes, usually in the 

 form of a loop, and always fixed to the dorsal valve (fig. 280). 

 In the genus Terebratula (fig. 281) itself, and in Terebrat- 

 ulina, the loop supporting the arms is very short, the former 

 commencing in the Devonian period, the latter in the Oolitic, 



Fig. 280. Terebratula sacculus Car- 

 boniferous. The right-hand figure shows 

 the interior of the dorsal valve with the 

 loop. (After Dawson.) 



Fig. 281. a, Terebratula quadrifida Lias ; b, Terebratula sphceroidalis Inferior Oolite; 

 c, Terebratula digona Bradford Clay, Forest Marble, and Great Oolite (Jurassic). (After 

 Davidson.) 



and both being represented by living forms. In the genus 

 Waldheimia (fig. 278) there is a very long loop, which is 

 bent backwards, and the same is the case with Terebmtdla 

 (fig. 276). The former appears to commence in the Trias, 

 the latter in the Cretaceous rocks, and both have survived to 



