110 CRETACEOUS LAMELLIBRANCHIA. 



angle ■\vitli the posterior margin, which is more or less truncated. Umbones 

 inconspicuous. Lunule smooth, ovate, rather broad, with a sharp edge. Escutcheon 

 smooth, deep, long. 



Ornamentation consists of 7 to 9 sharp, prominent, concentric ribs, with steep 

 dorsal and gentle ventral slopes. Interspaces l)road. Between the ribs there are 

 three or four very small concentric ribs and numerous fine radial strife. 



Measurements .- 



Length 

 Height 



Affinities. — This species resembles Astarfe frnnosa (see p. 112) Ijut is larger, 

 less triangular in outline, with a larger apical angle and fewer ribs. yl. si)nilis, 

 Goldfuss,' belongs to the same group, l)ut is more rounded in outline, and has more 

 numerous ribs than A. siiljcostata. 



Forbes states that English examples of this species were identified by d'Orbigny 

 as /]. nnmisvialix, d'Orbigny,- but adds that they resemble much more nearly 

 d'Orbigny's A. striato-costata. They differ from the figures oi A. numismalis given 

 by d'Orbigny in being less triangular, and in having the posterior end more 

 truncated ; also the ribs are less numerous but more prominent, and form a larger 

 angle with the postero-dorsal margin. English examples were also seen by Pictet and 

 Renevier, and were identified by them with Astarte laticosta, Deshaycs (= striato- 

 costata and suhcnntata, d'Orbigny). The specimen figured by d'Orbigny is more 

 elongate than most of the English examples, but agrees in this respect with some 

 found in the Atherfield Clay of Sevenoaks. The specimens figured by Leymerie 

 are much larger than the English examples, and the ribs are more numerous and 

 less prominent. Some of the differences seen in the figures of the authors quoted in 

 the synonymy are probably due to the differences in the age of the specimens. In 

 the young specimens the posterior end is more rounded, in old specimens it is 

 more truncated. 



Bemarl-s. — Specimens which occur in the Crackers of Atherfield (Plate XV, figs. 

 1 , 2) differ slightly from those described above, but probably constitute only a local 

 variety ; the valves are not quite so convex, the ribs are rather more numerous and 



' Tor figures and references see Holzapfel, ' Die Mollusk. Aachen. Kreide ' (Palaeontographica, vol. 

 XXXV, 1889), p. 194, pi. xix, figs. 11—15. A. similis was referred to the genus Gouldiahy Stoliczka : 

 Holzapfel discusses this subject, and I am in agreement with his conclusions. 



- ' Pal. Pran9. Terr. Crut,,' vol. iii (1844), p. 03, pi. cclxii, figs. 4—0; Pictet and Campichc, 'Terr. 

 Crct. Ste. Croix' (1800), p. 309. 



