PROTOSCHIZODUS IMPRESSUS. 233 



occupies the greater part of the valve. This condition causes a difficulty in deter- 

 mining, at first sight, to which side the valve belongs; but the slight traces of a 

 keel on the dorsal slope settle this question. The labels now on the specimens 

 are somewhat confused, but there is no doubt as to which specimens were the 

 originals of Portlock's drawings. P. carbonarius, the type specimen of which is 

 refigured, PI. XVII, fig. 11, has characters midway between P. axiniformis and 

 the variety depressus. 



Protoschizodus impressus, de Koninck, 1885. Plate XVIII, figs. 7 — 9 ; Plate 



XIX, fig. 1 . 



The " Cockle," TJre, 1793. Nat. Hist. Eutherglen, p. 310, pi. xv, fig. 2. 

 Dolarra securiformis, de Byckholt, 1853. Melanges Paleont., pt. 2, p. 83, 



pi. xvi, figs. 5, 6. 

 Axintts carbonarius, Gray, 1865. Biograpli. Notice of Bev. David Ure, p. 52. 

 Protoschizodus impressus, de Koninck, 1885. Ann. Mus. Hist. Nat. Belgique, 



vol. xi, p. 128, pi. xxii, figs. 

 19—24. 

 Compare — Halli, de Koninck, 1885. Ibid., p. 127, pi. xxii, figs. 11 and 12. 



— uncinatus, de Koninck, 1SS5. Ibid., p. 128, pi. xxii, fig. 18. 



Specific CJiaracters. — Shell of medium size, subtrigonal, somewhat oblique, 

 compressed, inequilateral, transverse diameter longer than the vertical. The 

 anterior end is depressed, deep in a dorso-ventral direction ; its upper border, 

 considerably below the level of the umbones, with a regularly rounded anterior 

 border, the curve of the lower segment being less rapid than the upper, and 

 therefore the lower part of the border is somewhat oblique from above downward 

 and backwards. The inferior border joins the anterior without a break, and is 

 curved, being convex downwards in front ; but the posterior moiety is almost 

 straight, and is directed upwards and backwards to join the posterior border, 

 with which it makes a bluntly rounded angle, a little less than a right angle. 

 The posterior border is oblique and almost straight, truncated from above 

 downwards. The hinge-line is arched, its extent not clearly marked off from the 

 anterior border in front, but behind it makes a rounded obtuse angle with the 

 posterior border. The umbones are subcentral, obtuse, gibbose, twisted inwards 

 and forwards, raised above the hinge-line, and not contiguous. The anterior edge 

 of the umbo rises from the body of the shell only just below the hinge-line ; the 

 posterior edge is continued obliquely downwards and backwards towards the 

 postero-inferior angle as an obtuse rounded ridge, which gradually becomes lost 

 on the surface of the valve. Posterior to the ridge the valve is compressed 

 rapidly to form the dorsal slope. The greatest gibbosity is subumbonal. 



Interior. — The anterior adductor muscle-scar is small, shallow, marginal, and 



30 



