On a New Exogyra from Del Rio Clay 17 



DESCRIPTION OF SPECIES 



Exogyra Cartledgei nov. sp. 

 PI. I, fig. 7-13; PI. II, fig. 1-4; PI. Ill, fig. 1-8. 



Shell medium-sized, very thick, elongate to sub-oval, some- 

 what oblique up to the beak, which is spirally coiled down- 

 ward and outward. 



Inferior valve. 



Shape very variable, elongate to obliquely suboval up to 

 the beak, strongly convex, the highest part forming an 

 obtuse, curved, umbonal ridge which follows approximately 

 the middle line of the height of the shell, and which disap- 

 pears toward the inferior margin of the valve. The slope 

 toward the anterior margin is sometimes much steeper than 

 toward the posterior side. 



Beak generally spirally twisted or coiled downward and 

 outward; sometimes pressed against the body of the shell, 

 sometimes almost entirely free in the shape of a ramshorn. 

 The beginning of the beak is never free, but always pressed 

 against the shell, which shows that in the juvenile stage 

 the beak was not detached from the rest of the valve and 

 that the development in the form of a ramshorn is the 

 latest stage caused by the spiral coiling of the valve during 

 the later stages. The point of the beak often shows a scar, 

 due to its having been attached to some small object, such as 

 shells, etc. 



The ornamentation consists of about fifteen coarse long- 

 itudinal ribs which on each of the slopes (anterior and 

 posterior) are nearly parallel on the upper half of the valve. 

 The series on the anterior side is not parallel to that on the 

 posterior one, but forms an angle of about 40 degrees. On 

 the umbonal ridge the two series are united by an inter- 

 vening space covered by intercalated, bifurcating ribs. The 

 ribs on the anterior side form a wide angle with the axis 

 of the umbonal ridge but on the last part of the beak they 



