ORTHIDtE. 263 



tooth [cardinal process], and two short, triangular, strongly diverging impressions of the 

 cardinal teeth [cardinal processes], immediately within and in front of which are the two 

 obtuse, obliquely conical bosses produced by the muscular impressions, separated by an 

 obtuse, short sulcus from a rounded mesial ridge, not defined in front or at the sides. 

 Anterior pair of impressions invisible; surface sharply radiated with the external ridges." 

 (M'Coy.) 



Obs. I know nothing of this species, and it is ignored in the table of ' Siluria ;' I 

 am, therefore, obliged to reproduce Prof. M'Coy's description and figures. He further 

 observes — " This species varies so much in form from compression in the schistose rocks 

 that I can scarcely venture to give the proportions, but I believe the true form to be 

 pretty nearly such as I have figured in the above work, namely, about three times wider than 

 long [?], with a straight front margin and nearly rectangular sides ; but some specimens 

 [distorted by pressure ?] are as long as wide. The average length is about seven 

 lines. This species is very easily recognised by the coarse, frequently branched, 

 twig-like ridges in all the middle part of the shell, the lateral ones being more nearly 

 straight, smaller, and frequently simple, and internally in the great length and distinctness 

 of the prominent muscular impressions- The great number of the ridges separates it 

 from the 0. Actonice, to which, however, it has no real affinity in any of its internal or 

 external characters. It is extremely common in the Bala Schists of Llyn Ogwen, North 

 Wales." It seems to me questionable whether this so termed species can be retained, but 

 for reference sake I have considered it desirable to reproduce M 'Coy's statement and 

 figures. 



OrTHIS iEQUIVALVlS, BttV. PI. XXX, figs. 9, 10. 



Orthis jEQUIValvis, Da«. London Geol. Journal, pi. xxvii, fig. 5, 1847; and Bull. 

 Soc. Geol. de France, 2nd ser., vol. v, p. 321, pi. iii, fig. 14, 



1848. 



Spec. Char. Shell almost circular, broadly rounded anteriorly, as well as at the 

 cardinal extremities. Valves almost equally and moderately convex, without fold or sinus ; 

 hinge-line shorter than the breadth of shell; areas narrow, that in the ventral valve 

 slightly the largest. External surface marked by numerous thread-like radiating striae, 

 which increase in number by the interpolation of a finer rib between each pair of the 

 larger ones. The whole surface is likewise traversed by concentric lines. 

 Length 10, width 11, depth 5 lines. 



Obs. This shell was named and figured by myself in 1847 ; subsequently another 

 (American) shell received from Prof. J. Hall a similar denomination. Orthis (sqidvahis is 



