EXOGYRA. 409 



Noil 1827. Chama conica, 8. Nilsson. Petrific. Suecaua, p. 28, pi. viii, fig. 4. 

 — 1837. — — W. Hieinger. Letha^a Suecica, p. 63, pi. xix, fig. 4. 



? — 1841. Exogtea conica, F. A. liiimer. Die Versteiii. d. nord-deutsch. Kreidegeb., 



p. 47. 

 ? — 1883. — A. Fritsch. Stud, im Gebieie d. biihin. Kreidef. iii 



Iserscliiclit., p. 117, fig. 92. 



Description. — Left valve very convex, with a more or less subtriangular or 

 semi-oval outline, very inequilateral. Anterior and ventral margins rounded; 

 posterior margin oblique, either straight, slightly concave, convex or sinuous, often 

 forming a rounded angle with the ventral margin. Umbo spiral, curved inwards, 

 and considerably backwards ; the attached surface usually behind the umbo. A 

 carina, generally distinct but sometimes rounded, extends in a curve from the umbo 

 towards the postero-ventral extremity. The part of the valve behind the carina 

 is more or less flattened, and may be concave near the umbo ; the part in front of 

 the carina is regularly convex. In large specimens the postero-ventral part 

 is more extended and the length of the shell in relation to the height becomes 

 relatively greater, the postero-ventral extremity is more rounded, and the 

 carina becomes less distinct. The surface of the valve is usually smooth, except 

 for growth-lines, but in some specimens (undata form) numerous somewhat 

 irregular radial ridges are present, either in the neighbourhood of the umbo or 

 over the entire valve. On the inside of the valve, at a short distance from the 

 margin, is a band of small transverse ridges and pits which broadens and becomes 

 more irregular near the postero-dorsal margin. Adductor impression rather near 

 the postero-dorsal margin, with its upper boundary straight or slightly convex. 



Right valve thin, operculiform, often slightly concave, with a small spiral 

 umbo ; surface nearly smooth ; outline varying with that of the left valve. 



In the forms described above (conica type), the surface of attachment is 

 small, but it may be very large so that all the left valve with the exception of a 

 narrow marginal part is fixed (ludiotoidea and Rauliniana types) ; between these 

 extremes every gradation is found. The free marginal part, which seldom extends 

 to the posterior border, usually grows nearly vertically upwards from the body to 

 which the shell is attached. When the attached surface is mainly or entirely 

 behind the position of the carina, then the free marginal part in front is larger, and 

 usually the shell is relatively higher and the spire of the umbo less developed, and 

 in such cases the marginal part sometimes forms an acute angle with the attached 

 surface. 1 The character of the surface (flat, concave, convex or irregular) to which 

 the left valve is attached also influences the form of the shell. 



Affinities. — The forms of small or moderate size, which are abundant in the 



1 See d'Orbiguy, pi. cccelxxviii, figs. 1 — 4. 



