46 EOCENE MOLLUSCA. 



" {a) Costre 24, 25, round, finely imbricated with intermediate costella3, imbrications 



continued over the interstices. Interstices deep; shell sub- 

 orbicular. 



(b) Costac 24 — 28, round, imbricated ; imbrications more distant than in a ; 

 interstices shallow ; costellse occasional ; shell tranversely sub- 

 orbicular. 



{c) Costae 18 — 32, round, finely imbricated; costellse occasional; shell nearly 

 orbicular. 



(d) Costa3 acute. 



(e) Multistriatus ? 

 (/) Tripartitus ? 



(g) Transversely sub-orbicular, costa3 30, coarsely imbricated. 

 {h) Costae round, smooth." {Edwards.) 



My own observations pretty nearly coincide with the above, and my formula will 

 stand thus : 



a. Ribs 25 — 32, rounded and smooth, without an intermediate ray. The smooth- 



ness may possibly be from attrition or decortication. 



b. Ribs 24 — 32, rounded and imbricated, without intermediate ray. 



c. „ 25 — 28 „ „ with „ 



d. ,, 24 — 28 tripartite, imbricated all over, without intermediate ray. 



e. „ 24—28 „ „ with 



/. „ 26 — 28 acute, subcarinate, and slightly imbricate, without intermediate 

 ray. 



Although we have both attempted the above divisions, I fear the lines of separation 

 will be anything but distinct between these variations. 



One perfect specimen, with the two valves united, shows the ribs quite smooth and 

 rounded ; another, equally perfect, has the younger portion of the shell with smooth ribs, 

 while the outer and older portion is strongly imbricated both over the ribs and between them j 

 and on another specimen the younger portion is imbricated, and the older portion smooth, 

 uniting thus in one individual the characters of what are called varieties. Where the 

 intermediate ray is developed, it is generally most prominent at the margins. I believe the 

 figures of Mr. Sowerby, above referred to, are representations only of varieties of this 

 variable species. The kO-radiatus I imagine to be one of the lesser number of ribs, with 

 the intermediate ray elevated into a primary one. The diameter of the shell is gem redly 

 a trifle in excess, measuring from the pedilateral to the siphonilateral margin ; but in some 

 specimens the proportions are decidedly reversed. The auricles are rather large and 



