STREBLOPTERIA LAEVIGATA. 49 



Specific Characters. — Shell above medium size, obliquely suborbicular, expanded 

 anteriorly, flattened and produced at the posterior superior angle. The left valve 

 convex, the right less so. The margin forming a single varying curve from the 

 anterior ear to the posterior superior angle, but the posterior margin having a less 

 curvature than the right. The hinge-lino straight, very short in front and pro- 

 longed backwards. The anterior ears small and triangular, compressed and 

 sharply defined, that of the right valve deeply slit for the byssus. No posterior 

 ear, but a rectangular expanded wing-like process. The umbones tumid, pointed 

 and incurved, gibbose, with a more sudden slope in front than behind, subcentral. 



Interior. — Unknown. 



Exterior. — The surface is smooth, but the microscope shows very fine con- 

 centric lines of growth, and the anterior ears have fine concentric, almost obsolete, 

 intricate markings. One specimen shows about 8 — 10 narrow, widely separated 

 radiating colour-bands. 



Dimensions. — PL XI, fig. 4, from Lowick, in the Woodwardian Museum, 

 measures — 



Antero-posteriorly . . . .97 mm. 



Dorso-ventrally . . . .97 mm. 



Localities. — England : the Carboniferous Limestone of Lowick and Settle. 

 Ireland: the Carboniferous Limestone of Caherass and Doohylebeg, co. Limerick ; 

 Cork, co. Cork; Millicent, co. Kildarc ; Cloghran, co. Dublin. 



Observations. — This species must be regarded as the typo of Streblopteria, 

 M'Coy, and demonstrates the absence of a posterior ear, so that the genus must be 

 limited to shells with this character. 



A very large specimen from Lowick (PI. XI, fig. 1) is perfect except the 

 anterior ear, but shows very well the character of the posterior superior angle 

 and hinge-line. It is a right valve. The left valve is more gibbose than the right, 

 and its anterior ear comparatively more depressed. The hinge-plate has not been 

 exposed, and I am unable to say anything on this subject. In young examples 

 the valve is comparatively more gibbose and the umbonal gibbosity more oblique. 



PI. XI, fig. 6, represents a specimen which has the colour-bands preserved. It 

 is from the dark limestone of Cloghran, co. Dublin. The opposing valve of the 

 same specimen (PI. XT, fig. 5) shows the anterior ear, the shell posterior to the 

 umbo having been broken. 



