122 SYNOPSIS OF THE CHARACTERS OF THE 



Tills elegant species is very rare in the Irish limestone series, where it has been occasionally confounded 

 with the O. pecteu, which it much resembles. Length one inch nine lines, width one inch eight lines. 



Ortiiis arcuata. Phil. 



Orthis arcuata. Phil. Pal. Fos. 



Sp. Ch. — Transversely oval; twice as wide as long, flattened; beaks small, prominent; ventral valve with 

 a slight mesial hollow ; surface with fine, divaricating striae, much curved at the sides. 



This species is rare; it is wider, and the strias are more curved than in O. interlineata. Length six lines, 

 width eleven lines. 



Orthis Bechei. M'Coy. PI. XXII. fig. 3). 



Sp. Ch. — Semicircular, depressed; nearly twice as wide as long; front straight, slightly convex ; sides 

 abruptly rounded, nearly straight, forming rectangular cardinal angles ; hinge-line equal to the width of the 

 shell ; cardinal area wide, triangular ; surface closely radiated with fine, very much branched, equal striae 

 (about twenty-six in half an inch of the margin); intervening spaces narrow, concave, sharply striated across. 



This fine species differs from the O. crenistria, in being so much more finely striated, and the striae 

 being equal in thickness, although, of course, very unequal in length, from their increasing so much in number 

 towards the margin, and from the O. arachnoidea, Phil., in its great width, transverse striation, cardinal 

 area, &c. From the nature of the striation, fragments are liable to be confounded vi\t\).\\vi Atrypa desquamata. 

 From the O. coma t a, M'Coy, it is easily distinguished by its sharp, straight, frequently branching striae, and 

 their greater distance ; the striae in that species being remarkably obtuse, close, and flexuous, very seldom 

 branched, and much finer, there being about eleven striae in a quarter of an inch at the margin of the O. Bechei, 

 and twenty-one in the same space of margin of tlie O. comata. Length one inch three lines, width two 

 inches. Dedicated to Sir Henry T. De la Beche. 



Orthis CADucA. MCoy. (PI. XXIL fig. 6). 



Sp. Ch. — Orbicular; perfectly flat, thin, width slightly exceeding the length; hinge-line shorter than 

 the width of the shell ; surface radiated with very numerous, fine, dichotomous striae, of unequal length and 

 fineness, slightly arched at the sides ; intervening spaces flat, finely striated transversely. 



This beautiful shell is distinguished from Captain Portlock's 0. cancellata, by its short hinge-line, more 

 numerous and finer striae ; the transverse strias being much more delicate in the present shell. Length eight 

 lines, width ten lines. 



Orthis circularis. M'Coij. (PL XX. fig. 19). 



Sp. Ch. — Outline circular; hinge-line less than one-third the width of the shell; valves nearly equal; 

 very slightly convex, radiated with thick rounded ridges, which are decussated by delicate, transverse striae, 

 and numerous, strong, scale-like lines of growth ; besides these there are a few scattered erect scales on the 

 ridges. 



This little species is remarkable for its almost perfect circularity of outline ; the valves are but slightly 

 convex, coarsely ribbed ; the beaks are very small, but prominent ; a few irregular, erect scales on some of 

 the ribs give an irregularly tubcrculated appearance to some specimens. Length five lines, width the same. 



Orthis COMATA. 3PCoy. (PI. XXIL fig. 5). 



Sp. C/;.— Truncato-orbicular; hinge-line as wide as the shell, extremities rectangular; cardinal area large, 

 triangular; beaks large, prominent; surface marked with very numerous, fine, longitudinal ridges, or striae, ob- 

 tusely rounded, close, equal, and slightly waving, very finely striated across. 



