Vol. 60.J RH.ETIC OF THE SOUTH-WALES DIRECT LINE. 2<j3 



resemble A.solitaria, Moore, in the absence of intercalation ; but, in 

 all adult forms, intermediate ribs make their appearance. 



Fragments of shell are found in the Bone-Bed, which exhibit the 

 characteristic ribbing of Avicula eontorta ; but, since they occur in 

 close juxtaposition to fragments of the Plkatula described on p. 202, 

 it is a little doubtful whether they may not be small portions of the 

 more strongly- and continuously-ribbed part of that shell. The weight 

 of evidence seems, however, against this view, as I have never seen, 

 on any specimen of the Plkatula, any ribs which run for so long a 

 distance without forming spines. Hence we may say that Avicula 

 eontorta extends downward into the Bone-Bed. Upward it occurs, 

 very rarely, just beneath the Estheria-hed. 



Avicula fallax, Pniicker=J/o/M)tts decussata, auctt. \_non Minister, 

 fide Brauns (5)]. 



We have found one or two specimens, as already noted, in the 

 uppermost beds. 



Modioli sodburiexsis, sp. nov. (PI. XVIII, tigs. 13 & 3 a.) 



The largest dimension varies from 5 up to 19 millimetres. 



The shell is extremely thin ; both valves are exactly similar : in 

 the young form the valves are strongly convex, but in the adult 

 they become natter. 



The beaks are close to the front end, and there is a slight indenta- 

 tion just in front of them. Behind the beaks the hinge-line 

 ascends straight and obliquely, and the valves are broadest where 

 the straight hinge-line merges into the posterior curvature. 



The front margin is uniformly rounded, and only projects slightly 

 in front of the beak. The posterior margin is also uniformly 

 rounded, though in adult forms it projects slightly more near the 

 base. The lower margin is alwavs convex, but becomes nearly flat 

 in the adult. 



The young form is almost perfectly oval in contour, the beak 

 small, the hinge-line short, and the interior almost smooth. 



In the adult, concentric growth-lines are well-marked, and a few 

 faint radial striae can be made out. A scarcely-perceptible ridge 

 runs from the beak, diagonally backward, across the valve, but 

 there is no distinctly-separated, swollen, anterior portion below it. 



The interiors (which are extremely abundant) show no trace of 

 pallial line, muscular impressions, or teeth. 



Figs. 12, 13, 27, & 33 (pars) in pi. i of 'Der Jura,' all bear a strong 

 resemblance to our form, but the peculiarity of the hinge-line is 

 best expressed in tigs. 12 & 27. Of these figures, Quenstedt 

 {op. tit. pp. 29, 30) remarks that rigs. 12 & 13 may belong to the 

 Lithophagi, and that fig. 27 recalls Astarte obliqua. The absence 

 of teeth and the thinness of the shell, as well as the straisrhtness 

 of the hinge-line, remove our species from Astarte ; while the 

 manner of occurrence prevents its inclusion among the borers. 



Abundant in a saudv micaceous bed, near the maximum of 

 At 'tenia eontorta. 



