REPORT ON THE OTRBIPEDIA. 1 :!.""> 



with a kind of radius, which projects over the immovable tergum. The articulation of 

 the rostrum with the fixed scutum is the leasl distind of all. 



When the operculum is taken away, the orifice resembles a scalene triangle; one side 

 (the shortest) is formed by the occludenl margin of the fixed tergum; the second side 

 partly by the occludent margin of the fixed scutum : the third (the Longest) side 1>\ the 

 upper margin of carina and rostrum combined. 



The movable scutum (PL XII. figs. 2 and -1) is nut very small, its area equalling nearly 

 two-thirds of that of the movable tergum. Its shape is triangular; the tergal and basal 

 margins are about at right angles to each other, and the occludenl margin is curved. The 

 length of the basal margin is not quite half the length of the tergal margin. The valve 

 is very thick, especially towards the apex; when investigated from the under side, the sur- 

 face appears to be bordered by a well-developed rim along the occludenl margin. This 

 rim separates a rather deep depression tor the adductor muscle from the occludenl margin. 

 Towards the sharply pointed apex the rim slightly increases in width. The tergal margin 

 is straight; the upper articular ridge is hardly visible, most probably it is represented by 

 the edge of the valve itself, which forms a furrow with a very short ridge at the under 

 side of the valve near the apex : the second (lower) articular ridge, on the contrary, 

 is well-developed, and is almost parallel to what 1 propose to consider as a third articular 

 ridge. The latter, which can he also regarded as an axial ridge, runs from the apex t<. 

 the basi-tergal corner of the valve, is very prominent, grows slightly broader towards the 

 under extremity, and here projects slightly. It corresponds exactly to the third 

 articular ridge of the tergum, which is also considered as such by Darwin. The second 

 articular ridge describes a curve, and has a narrow semi-circular part of the valve between 

 itself and the tergal margin. In this part of the valve the lines of growth are very 

 oblique on the tergal margin of the valve. The part of the valve which is enclosed 

 between the third articular ridge ami the occludent margin firms the greatesl half 

 of it, and has the ridges of growth in the inferior part parallel to the basal margin : 

 towards the upper extremity the ridges are slightly divergent. 



The movable tergum (PL XII. figs. 1 and .3) is broad and quadrangular. The valve is 

 divided into two triangular parts by the beautifully curved axial or third articular ridge. 

 In that part of the valve which lies between this ridge and the occludent margin, the lm -. 

 of growth are parallel to the basal margin; in the other half these lines are parallel to the 

 scutal margin. The axial ridge widens downwards and projects very dial inctly at t he basal 

 point of the valve. The middle articular ridge is close to the axial ridge, and is separated 

 by a somewhat greater distance from the upper, or first articular ridge. Its free edge 

 is formed by the occludent margin of the valve, hut it grows broader towards tin' scutal 

 margin, where it is produced into a slight projection. The occludent margin of the 

 valve consists of two parts, separated by the umbo or apex of the valve, — one beneath 

 the apex (and this part is curved) and the other between this apex and the scutal 



