PROTHYRIS. 87 



convex growth-swellings, separated by deep impressed lines, and apparently 

 covered with finer lineations, and with one or two impressed rays on the postero- 

 superior slope. Contour flattish on the back, steeply convex on the supero- 

 posterior part, where a narrow groove runs from the rear of the umbo to the 

 margin, and broken in front by an arching ridge running from before the umbo to the 

 anterior marginal nick, and defining a lunule. Shell-structure massive. Pallial 

 line at some distance from the margin. Posterior muscle-scar apparently small, 

 and situated near the posterior point. Anterior muscle-scar near the umbo. 

 Interior of the shell with grooves and pits. 



Size. — Height 12 mm., length 18 mm., depth of one valve about 3 mm. 



Localities. — There are twenty-six specimens in the Museum of Practical 

 Geology from Baggy Point, " South Cave," and " just over the grits " in blackish 

 Limestone. 



BemarJis. — Though these shells were evidently common in their locality, it has 

 been difficult to find sufficiently perfect specimens to define the species, especially 

 as it seems to have been rather variable in shape. 



Affinities. — It differs from Prothyris contorta in being shorter and less oblique, 

 and in having a convex base. 



2. Peothyris contoeta, n. sp. Plate IX, figs. 15, 16, 16a. 



Description. — Shell small, convex, subquadrate, very transverse. Hinge-line 

 unseen, but apparently about half the length of the shell. Umbo small, low, 

 oblique, flattened, and situate near the anterior end. Anterior margin very 

 convex, but broken at about its centre by a small, deep, concave notch, which is 

 the termination of a rounded elevated conical ridge, running to it from the front 

 of the umbo. Inferior margin very long, convex in front, straight or even 

 slightly concave in the median region, becoming convex behind. Postero- 

 inferior corner produced and very deeply convex. Posterior margin oblique 

 above. Contour of back very evenly convex except for the tubular ridge. Cast 

 marked with one or two deep linear furrows running along the postero-superior 

 edge, and with a triangular perpendicular clavicular groove under the umbo. 

 Muscle-marks apparently large and not marginal. Pallial sinus apparently deeply 

 indented. Surface covered with very numerous microscopic, impressed, con- 

 centric striae, and having five or six fine sharp sub-parallel ridges running along 

 the postero-superior side of the shell immediately below the hinge-margin. Shell- 

 structure rather thick for the size of the shell. 



