ORTHIDtE. 275 



OrTHIS ? SPIRIFEROIDES, M'CoTf. PI. XXXVII, figs. 3—7. 



Stropiiomena. SPIRIFEROIDES, M'Coij. Anuals Nat. Hist., vol. viii, p. 402, 1851. 

 Leptj^ka — Id. Brit. Pal. Foss., p. 246, 1852. 



Stropiiomena — Salter. Siluria, 2ncled., p. 211, fig. 2, 1859 ; but in the 



table, p. 544, it is quoted as an Orthis. 

 Orthis — Id. Mem. Geol. Survey, vol. iii, p. 2G8, 1866. 



ISpec. Char. Sub-rhomboidal, wider than long; hinge-line straight, nearly as long as 

 the shell is wide; cardinal angles obtuse; sides rounded; front abruptly raised. Dorsal 

 valve semicircular, rather deeper or more convex than the opposite one, and divided 

 along the middle by a rather wide, convex, mesial fold ; hinge-area linear. Ventral 

 valve moderately convex, with a wide, sharply defined, flattened sinus ; area triangular, 

 narrow ; fissure open (?) ; beak small, incurved. Surface of both valves closely covered with 

 numerous, very fine, radiating, thread-like, raised strise, which increase in number 

 as they near the margin by bifurcation and by the interpolation of smaller striae. 

 In the interior of the ventral valve the hinge-teeth are supported by small dental plates, 

 which enclose two elongated, oval, muscular scars, divided longitudinally along the 

 middle by a narrow flattened median ridge. The adductor occupies the greater part 

 of the middle or flattened ridge, and is flanked by two large divaricator impressions ; and 

 again, close to the dental plates, and on the outer side of these last, are two smaller 

 scars, probably referable to the ventral adjusters. Above the adductor scar, or between 

 it and the beak, rises a small cylindrical curved tube. In the interior of the dorsal 

 valve a small bifid cardinal process is situated between the hinge-sockets, and under it 

 the quadruple impression left by the adductor extends to about half the length of 

 valve. 



Length 10, width 12, depth 7 lines. 



Ohs. I feel greatly puzzled as to the genus to which this abnormal form should be 

 referred. It is certainly neither a Stropiiomena nor a Leptmia, and I question very 

 much if it be an Orthis. It difiers in some respect from Spirifer, although not 

 nearly to the extent imagined by Prof. M'Coy and Mr. Salter ; nor would I, indeed, 

 feel surprised if the future discovery of spiral appendages should oblige us to refer 

 it to the last-named genus. The shape and position of the muscular scars have more the 

 character of those of Spirifer than of Orthis. I will, however, follow Mr. Salter by 

 provisionally leaving it with Orthis, as I cannot state affirmatively that it is a Spirifer. 

 Messrs. M'Coy and Salter observe that it is so like Spirifer radiatus of the Wenlock 

 Limestone as to be often mistaken for it ; but it may be distinguished from that shell by 

 a longer hinge-line, flatter sides, and coarser striation. One of its most interesting 

 internal characters consists in the presence of a small, cylindrical, curved tube (fig. 5), 



