ORTHIDiE. 253 



margin. In the interior of the dorsal valve the cardinal process is situated between two 

 small brachial processes, on the outer side of which are placed the hinge-sockets ; the 

 quadruple impression of the adductor muscle is divided by a longitudinal ridge, and each 

 pair again divided by a small oblique ridge. 



Length 12, width 17, depth 4 lines. 



Obs. In 1839 Mr. J. de C. Sowerby described this important Lower-Silurian species 

 as " transversely obovate, with about fourteen large radii, bifid or quadrified at their 

 extremities; one valve flat, the other very convex. Length 8^, width 11 lines. Greatly 

 resembling O.jlahhelhdwm, but known at once by the forked radii, a character particu- 

 larly useful in distinguishing the species among the slaty rocks." Murchison's specimens 

 were incomplete and not over-well preserved casts, so that the original describer of the 

 species had not the means of making known all the characters pertaining to his shell, and 

 which subsequent observers have described. Had Sowerby's material been more abun- 

 dant he would have seen that the number and strength of the ribs varied exceedingly in 

 different specimens, from fourteen to thirty or upwards being at times present round the 

 margin. The smaller or dorsal valve is sometimes flat, but in well-preserved examples it 

 is often shghtly concave and especially so near the umbone, and this concavity is the 

 normal condition of the valve, and is not the result of any violence, as suggested by 

 Prof. M'Coy to be the possible cause of the depression. Indeed, Orthis Adonia simulates 

 certain forms of Strophomena, but the internal characters determine it to be an Orthis. 



In his description of the fossils found by the Survey in North Wales, Mr. Salter 

 alludes to this species in the following words : — " Of this I thought it necessary to give 

 complete figures, and shall point out the characters on which we may rely to separate it 

 completely from O.jlahellulum. There is no danger of confounding it with any other British 

 species. First, the relative convexity and concavity of the two valves is different in each. In 

 . jlaheUidum the ventral valve is the flat or slightly concave one, in 0. Actonia the 

 reverse is the case. . flahellulum has a hinge-line shorter than the whole width of the 

 shell ; in 0. Actonia it is much wider, and produced into sharp ears. The former has 

 rounded, the latter sharply angular, ribs ; and lastly, in 0. flabelhdmn the ribs bifurcate ; 

 in 0. Actonia they are simply interlined by other ribs (which towards the margin give 

 rise to a cluster of small ribs). There are no longitudinal and but few transverse striae; 

 sometimes these last are sharp and distinct. But a few distinct varices of growth are 

 always present, and it is beyond the last of these that the sudden multiplication of the 

 ribs takes place which gives so curious an appearance to our fig. 3, and which has been 

 well figured in ' Siluria.' .... Thearege are moderate, and equal in either valve." 



Position and Locality. Very common in a great many locahties in Wales and the 

 South of Ireland in Caradoc rocks ; but in ' Siluria' it is stated to occur also in the Llan- 

 deilo and Llandovery formations. In the Upper Llandeiloitis found at Garn, Arenig. In 

 the Caradoc or Bala at Acton Scott, Church Stretton, in Shropshire, at Bryn-Bedwog near 

 Bala, and in many localities in that district ; Moelydd shales ; Castell Craig Gwyddon, 



