NOTES ON THE BRITISH JURASSIC BRACHIOPODA. 23 



The foramen is partly or entirely margined by one or two 

 plates called the deltidium. 



In other forms there exists a triangular fissure, which 

 either remains open or is partly arched over by a pseudo- 

 deltidium. In some other forms again, such as Gyrtia 

 exporrecta, Leptoena (Strophomena) rhomboidalis and others, 

 a circular foramen exists in the young individual at the 

 extremity of the beak or in somfe part of the arched delti- 

 dium, and continues open for some time, but becomes closed 

 or cicatrised in the adult (10, p. 289). 



The development, shape, size, and position of the beak, 

 foramen, and deltidial plates are all very important charac- 

 ters in the determination of species and varieties. 



Some genera, however, such as Lingula, have no perfora- 

 tion — the peduncle simply passes up between the apices of 

 the valves ; while others, such as Crania and Thecidea, 

 adhere to submarine objects by a portion of the surface or 

 substance of the ventral valve. Then again there are others 

 which were furnished with spines on the surface of the 

 valves, which they wound round various objects for attach- 

 ment, such as Etheridgma (Productus) complectens, and other 

 species. Others, which are believed to have been attached 

 in the young state, were free in the adult, such as some 

 of the Spiriferidai ; while some seem to have never been 

 attached at all (10, p. 289). 



The peduncle is seldom met with in the fossil condition. 

 The only instances of its occurrence of which I know are the 

 one or two cases recorded by Davidson in his '' Silurian 

 Monograph " and its Supplement, and a specimen of Ter. 

 punctata, var. haresjieldensis, which I found in the so-called 

 Cephalopod-bed of the Inferior Oolite at Wotton-under- 

 Edge, Grloucestershire. There may be, and most probably 

 are, other instances on record which have not vet come to 

 my notice. 



