5 -A PALEONTOLOGY 



the ventral .side the single attachment of the adductor muscles 

 in the centre, and on each side of it the cardinal muscles ; 

 these are surrounded by the punctate ovarian spaces and 

 impressions of the large pallia! sinuses. The genus Orthia 

 includes 1 00 species, ranging upwards to the Permian, but it 

 is most abundant in the Silurian rocks. Some of the lower 

 Silurian species have a round foramen in the " pseudo-delti- 

 dium," and are called Orthisince (d'Orb.) Other species in the 

 upper palaeozoic rocks have the beak twisted or deformed, 

 probably owing to the attachment of the shell when young 

 (= Strejrforhynclius, King). In Stro2)lwmena, Rafin (= Leirtcena, 

 Dalm.), there is a minute byssal foramen when young, of 

 which no trace exists in the adult ; and the deltoid notch is 

 also closed, except the space required to receive the divided 

 cardinal process of the dorsal valve. The oral processes 

 appear to be shifted to the centre of the valve. The shell, 

 when young, is plano-convex, but when it has attained a certain 

 size the valves are bent over to one side or the other, and more 

 or less suddenly. The pallial impressions are the same as in 

 Ortliis. 



The genus Davidsonia, peculiar to the Devonian limestones, 

 resembles an Ortliis attached, like Thecidium, by the ventral 

 valve to corals, and sometimes taking the markings of the 

 body on which it grows, like the oyster and Anomice. The 

 pallial impressions are like those of Ortliis, and the form of 

 the spiral arms is indicated by prominences which almost fill 

 up the interior of the shell in aged examples. Some indica- 

 tions have been obtained of slender calcareous spires for the 

 support of the arms in this genus ; and also in Koninckia, a 

 small shell from the trias of St. Cassian, in which there are 

 always spiral grooves in the interior of the valves crossed by 

 the impressions of the pallial sinuses. 



The anomalous fossil called Calccola sanded ina (Lam.) is 

 also peculiar to Hie I levoniau limestones. In shape it resembles 



