APTYCHOPSIS OBLATA. Ill 



There are two other carapaces, probably oblately circular in shape, when 

 perfect, which approach A. angulata. 



PL XV, fig. 13, consisting of one perfect and one imperfect valve, has been 

 somewhat shortened by crumplings, induced by pressure at right angles to the 

 dorsal axis. It may, therefore, have been more nearly circular, like A. glabra. 

 No trace of concentrics remain on the disturbed and somewhat pyritous surface. 



Measurements at present: | , 7 mm.; — , 8 mm.; /_, 5 mm. at 45°; ^_,, 

 8 mm. ; I, 5 mm. ; V , 90°. The apparent proportion of the width of the whole 

 carapace to its length is as 16 to 12. 



In black, finely micaceous shale. Streamlet, Craigdasher Hill, 4 miles W. of 

 Dunscore, Dumfriesshire. Mus. Geol. Survey Scotland. M 4516. 



PL XV, fig. 17, consists of two valves flattened and distorted by oblique 

 pressure, and represented by rusty films, with numerous concentrics almost 

 regular in width. 



Measurements : | , 14mm. ; — , 12 mm. ; A, 6 (?) mm. at (?) ; ^_^, 9 (?) mm. ; 

 I, 5 (?) mm. ; V (?)• The proportion of width to length of the perfect carapace 

 was probably about 24 to 19. 



In grey mudstone, dark-grey at heart, very finely micaceous, much squeezed, 

 having its fossil-bearing plane (the bed is one inch thick) at an angle of 50° with 

 the cleavage-plane. From the Brathay (Lower Ooniston) Flags, Nanny Lane, 

 Troutbeck, Windermere; collected by Mr. J. E. Marr, F.R.S. Woodwardian 

 Mus., Cambridge. 



It is referred to in the ' Geol. Mag.,' 1884, p. 355 ; the ' Report Brit. Assoc, 

 for 1884 ' (1885), p. 92 ; and the ' Catalogue of Type Fossils in the Cambridge 

 Museum,' 1891, p. 134. 



12. Aptychopsis OBLATA, sp. nov. Plate XV, figs. 18, 21, 23. 



Some extremely oblate, or quite transversely oval, carapaces have now to be 

 noticed. 



PL XV, fig. 21, has two valves each semi-elliptical transversely, together 

 making an oval, with a broad shallow notch between them anteriorly. The angles 

 below the notch retain a particular convexity, bounded by a distinct furrow 

 parallel with the almost obliterated usual concentric lines of growth, which seem 

 to have been of irregular widths, and to have ended at the outer margin with a 

 flattish rim. The central umbonal convexity in each valve looks like the area of a 

 certain stage of growth defined by some altered conditions of existence, as in the 

 valves of many Lamellibranchiate Molluscs. 



