ORTHIDyE. 279 



Orthis adscendens, Be Ferneuil and De Keyserl. Geol. Russia, vol. ii, p. 203, pi. 



xii, fig. 3, 1845. 

 Orthisina — M'Corj. Brit. Pal. Foss., p. 231, 1852. 



Oethis — jyEichw. LethaeaRossica, Periode Ancienne, vol. i,p. 838, 1859. 



Oethisina — Salter. Siluria, 4th ed., p. 527, 1867. 



Spec Char. Rotundato-quadrate, slightly wider than long ; sides almost parallel ; 

 front rounded ; hinge-line either less than or somewhat exceeding the width of the shell ; 

 cardinal extremities rectangular or slightly prolonged, with angular terminations. Ventral 

 valve convex or sub-pyramidal ; greatest height at the point of the beak ; without fold 

 or sinus ; area flat, about six times as high as long ; fissure wide, triangular, entirely 

 arched over by a convex pseudo-deltidium. Dorsal valve very moderately convex, much 

 more shallow than the opposite one; hinge-line linear or rudimentary, with a narrow 

 convex pseudo-deltidium closely juxtaposed to the one in the opposite valve. Surface of 

 both valves covered with numerous, radiating, rounded, bifurcating, raised strise, with 

 interspaces of almost equal width ; each valve crossed by concentric lines of growth, 

 more numerous and close as they approach the margin. 

 Length 12, width 13, depth 6 lines. 



Obs. This species has been well described by M. de Verneuil, Prof. M'Coy, and others. 

 Pander had given it a great many names, out of which that of adscendens has been 

 adopted by the generality of palseontologists. MM. de Verneuil and d'Eichwald are 

 likewise of opinion that the Orthis zonata of Dalman and Hisinger may probably be 

 referable to the same species. It is a variable shell, the dorsal, as well as the 

 ventral valve, being much less convex in some specimens than in others, and, according to 

 D'Eichwald, the dorsal valve is almost flat. There is also much difierence in the compara- 

 tive length of the hinge-line, which is shorter in some examples than in others. Prof. 

 M'Coy observes that " Our specimens correspond precisely with Pander's type-figure above 

 quoted ; but if, as M. de Verneuil supposes, the various other species on that and the 

 succeeding plate be only varieties '' [I should say mere modifications in shape, as occur in 

 every species], " it is obvious that there is considerable variety in the depth of the valves, 

 and that the cardinal area does not invariably lean forwards to the extent mentioned 

 above, but has sometimes even a slight backward inclination, as in the Orthida generally ; 

 and in this case, I think, there could be no doubt that the whole might be well referred 

 to 0. zonata of Dalman ; but as I have not examined that species, I am unwiUing 

 (like M. de Verneuil) to make an uncertain approximation." 



In Russian examples, as remarked by M. de Verneuil, all the external surface is 

 covered with widish striae and intervening spaces ; these ribs are bifurcated, round, 

 and scooped out like a tube, so that when they are worn on their surface, or obliquely 

 cut by friction, they form a series of festoons round the shell, and especially so when 

 the longitudinal lines, which cross the valves, protrude by their multiplication, in the 

 shape of terraces. None of the few English examples however I have seen show that 



