BRACHIOPODA. 55 



occasionally manifested in both valves of the species to form two or three 

 vaults on either side of the platform. In animals of the size indicated by these 

 shells the lobation of the liver may have been so strongly developed as to pro- 

 duce this peculiar effect in the excavation of these cavities. Admitting the 

 sequestration of the liver in Trimerella, a freer play would undoubtedly be 

 left to the muscles and greater opportunities of development, and this fact will 

 have had both a direct and inverse bearing on the size attained by both the 

 platform and the shell itself. 



In regard to the great umbonal chambers of the pedicle-valve in Monomer- 

 ELLA, and their lesser development in Dinobolus and Trimerella, they have pos- 

 sibly served a similar function to the platform-vaults, by removing the delicate 

 genital organs from dangers which might arise from the action of the muscles. 

 In Lingula anatina, these organs lie normally in the median and posterior portion 

 of the visceral region, pressing backward against the post-parietal wall, and are 

 placed mostly on the ventral side of the body. In offering an interpretation 

 of the arched posterior scar in Lingulops, Davidson and King suggested that the 

 middle sinus of the arch might have been produced by the umbonal muscle 

 pressing against the parietal wall, and the lateral sinuses have been due to the 

 pressure of the ovarian lobes against this muscular wall. Were this the case 

 in the genus where all these features are in an initial stage of development, it 

 would readily be conceived how by this constant pressure the genital organs 

 may have developed the umbonal cavities for their own protection. 



Genus LINGULELLA, Salter. 1866. 



PLATE II, FIGS. 1-13. 



1852. Lingula, McCoy. Annals and Magazine of Natural History, Second Sei-ies, vol. viii, p. 405. 



1852. Lingula, McCoy. British Palaeozoic Fossils, p. 252, pi. 1 l. 



1859. Lingula, Salter, In Murchison's Silui'ia, Fourth Edition, p. 43. 



1866. LinguleUa, Salter. Mem. Geological Survey of Great Britain, vol. iii, p. 383. 



1866. LinguleUa, Davidson. British Silui-ian Brachiojioda, p. 55. 



1867. LinguleUa, Salter. Quarterly Journal Geological Society, p. 340. 



1868. LinguleUa, Davidson. Geological Magazine, p. 303. 



1871. LinguleUa, Davidson. British Silurian Brachiopoda, p. 336. 



