BRITISH SILURIAN BRACHIOPODA. 



81 



Wincliell, and others to be present in the shell of Syringothyris. It is probable that the 

 fourth division, including M'Coy's genus Beticularia, should be retained for such shells as 

 8p. lineata, Martin, Sp. imbricata. Sow., Sp. Urii, Flem., Sp. lavis, Hall, 8p. curvata, 

 Schloth., and others, in which the shell-structure is perforated by canals, and the external 

 surface is closely covered with flattened spines, in some divided internally into two tubes ; 

 these will be found to have been minutely described in p. 275 of my 'Carboniferous 

 Supplement.' 



The position and direction of the spirals is also somewhat variable, even in specimens 

 of the same species. In the larger number of forms the extremities of the spiral cones are 

 directed towards the lateral portions of the shell ; but it has been noticed that in 

 species and specimens with a deep ventral valve and large rostral cavity the spirals, 

 in order to find more accommodation, extend to a considerable distance upwards into the 

 cavity of the beak, and on either side of the median septa. This is particularly observ- 

 able in some specimens of Cyrtina heteroclyta, and in those specimens of Jurassic Spiri- 

 ferince which are possessed of a large rostral cavity (see ' Oolitic and Liassic Suppl.,' PI. 

 XI, and ' Dev. Suppl.,' Pis. I & II). 



In all the genera and species of Spiriferida with which we are acquainted, and which 

 have been internally investigated, the principal stems of the spirals are simply attached to 

 the hinge-plate and run nearly parallel to each other until they 

 form their final convolution. 



As far as we are as yet aware, the two principal stems of the spirals 

 in Spirifera and in Cyrtia are unconnected. At about half their length 

 they give oS" two short lamellae, which extend only a little way into the 

 interior of the shell (see 'Sil. Suppl.,' PI. IV, figs. 10, 11). Mr. 

 Glass has experimented on a number of species and specimens, and has 

 never detected any connection. We do not know yet whether in 

 SyringotJiyris and Beticularia the principal stems are unconnected. 



In Cyrtina Mr. Glass has worked out their connection, which is a 

 simple V-shaped band ; and it appears to be the same in Suessia ; 

 while in Spiriferina it is in the shape of a curved lamella. lamella (a). 



Spiriferina rostrata, 

 showing the attach- 

 ment of principal 

 lamellae to hinge- 

 plate, and lower 

 down the connection 

 of the two stems by 

 means of a curved 



