arene 
1888. ] 139 [Meyer. 
II. Sprcirs New To THE AMERICAN TERTIARY. 
Caecum stevensonin. sp. Figure 4. 
Relatively large; regularly and well curved. Shell thick. Surface 
closely covered by strong longitudinal coste, which are usually alterna- 
ting. Septum mucronate. 
If the shell is kept horizontal, its convexity towards the observer, the 
mucronation of the septum appears on the left side. Its size and position 
is variable, sometimes it is nearly marginal, sometimes subcentral. The 
cost are usually broader than the interstices between them and alterna- 
ting, but in some specimens they are nearly equal. The younger part of 
two specimens shows indications of rings, producing a very slight can- 
cellation. There is a contraction at the aperture, but it is confined to the 
margin of the aperture and nearly imperceptible. Besides the regular 
form there occur smuller specimens of equal ornamentation, but different 
shape. They have a less cylindrical tube, the increase in width towards 
the aperture being much larger. The same small and tapering form occurs 
with the following species. Wood observed the same form occurring with 
the Crag species of Ceecum. He considered them at first as distinct spe- 
cies, and then described them as varieties (see Crag Mollusca, Vol. i, p. 
116, Pl. 20, fig. 40). I think they are the adolescent state of the species. 
I have not become aware of a form of the European Tertiary, with 
which to compare C. stevensoni, and no costated Cecum is known from 
the American Tertiary. Specimens of the recent Ceecum cooperi Smith 
were compared and proved to be somewhat similar, but specifically differ- 
ent. They have fewer and thinner ribs, are distinctly cancellate and have 
a different shape. CO. stevensoni is quite common in Yorktown. 
Cacum virginianum n. sp. Fig. 3. 
Relatively large ; regularly but slightly curved. Shell thick. Septum 
prominent, angular. Surface smooth, with concentric strie of growth. 
The mucronation of the septum is situated and is variable, like in the 
preceding species. Specimens of the adolescent state have a more taper- 
ing form, as in the preceding species. 
A similar European fossil is apparently Cecum mammillatum Wood,* 
from the Crag ; which, however, is more curved and has a less angular and 
more mammillated septum. About the differences with other smooth spe- 
cies of the American Tertiary see below. Similar recent species are appa- 
rently Caecum chinense de Folin + and Caecum leeve Adams.{ I have not 
been able to compare specimens of these two species. C. chinense, accord- 
ing to De Folin’s figure, seems to be more slender and more tapering and 
the septum seems to be different. C. lieve is described as shining and is 
apparently a shorter species. 
* Wood, Crag Mollusca, Vol. i, p. 116, Pl. 20, fig. 4. 
+ De Folin, Fonds de la mer, Vol. i, p. 80, Pl. 9, fig 3-4. 
t See Tryon’s Manual of Conchology, Vol. viii, p. 215, Pl. 66, fig. 52. 
