XOTIIAMl'SrUM TRANS VERSUM. Kil 



Observations. — I have only seen one English example (Plate XXIV, fig. 11), 

 and this is much larger than any of the Irish specimens, of which I have seen 

 at least ten. When the shell is young the radiating ribs are almost absent, but 

 they become better marked towards the inferior margin of the valve. The 

 radiating ribs are simple at first, but in old shells bifurcate as they approach the 

 lower margin. 



As I mentioned above, this species has generally been referred to Avicula 

 flabellula in Irish cabinets, but M'Coy gives the dimensions of his shell as length 

 7^ lines, width 9 lines, whereas the antero-posterior diameter of Nothamusiwm 

 radiatum is much longer than the dorso-ventral. I am not sure that N. radiatum 

 is distinct from N. transversum. Dr. Kitchin thought the two specimens belonged 

 to distinct species, but the variation in shape may be due to the fact that we are 

 comparing the cast of an interior with a testate example. The two examples, 

 however, which I refer to N. transversum have the antero-posterior diameter 

 comparatively longer than that which obtains in N. radiatum. 



Notiiamusium TiiANSVEiiSUM, sp. nov. Plate XXIV, figs. 12, 13. 



Specific Characters. — Shell of medium size, oblique, very slightly convex, 

 transversely ovate, truncate anteriorly, antero-superior angle produced and 

 pointed, anterior margin sinuous. Inferior margin broadly convex, posterior 

 border bluntly rounded. Hinge-line straight and long. The umbones are 

 moderately long, compressed and pointed, placed far forward. 



Interior. — The hinge of the right valve has a large, thick, cardinal tooth, 

 projecting obliquely backwards, above which is a narrow, linear sulcus, probably for 

 the attachment of an external ligament. 



Exterior. — The surface of the valve is smooth in the umbonal region and in 

 front, this part of the valve being marked by fine concentric lines of growth ; 

 elsewhere the valve is covered by numerous fine, flat, simple, rarely bifurcating 

 lines of growth. Shell thin. 



Dimensions. — Plate XXIV, fig. 13, measures — 



Antero-posteriorly . . . .62 mm. 



Dorso-ventrally . . . .37 mm. 



Localities. — England : the 4-laws Limestone, the Coomb, Redesdale, Northum- 

 berland. Scotland : the Lower Limestone Series of Auchenmade, Ayrshire. 



Observations. — Mr. J. Smith, of Kilwinning, sent me some years ago the shell 

 figured in PI. XXIV, fig. 13. It puzzled me until I came across the specimens of 

 Nothamusiwm radiatum from Little Island; and though Mr. Smith's specimen is 

 much worn, I have no hesitation in referring it to the same genus. This specimen 



