6o Bulletin 4 174 



ness of the ribs. Fig. i is from a specimen in my collection, the 

 other (lb) is in the State collection. Rather abundant. Named 

 in honor of Dr. Eug. A. Smith." 



The giving of this form a new specific name is justifiable or 

 not, according to one's idea as to what constitute specific differ- 

 ences. If one should confine his attention to a few specimens 

 collected at random from beds 18 and 19 at various places 

 from Prairie Bluff to Allenton, it would be possible generally 

 to say that certain forms have from 30 to 35 high narrow 

 crenulated ribs, while others have from 26 to 30 lower, 

 broader, smoother ones. Still the writer has experienced even in 

 this limited area and geological horizon considerable difficulty in 

 making this distinction. If, however, we widen our range both 

 geographically and stratigraphically matters begin to become more 

 seriously complicated. On the Chattahoochee river at the very 

 base of the Midway, the lower, broader ribbed and more typical 

 form is sharply marked with deep concentric lines. In the vicin- 

 ity of Crainesville, Tenn., the ribs though high, narrow and 

 crenulate, have decreased in number to 25 or 26. The specimens 

 collected by Dr. SafFord and referred to by Gabb (Jr. Ac. Nat. 

 Sci. Phila. , i860, vol. iv, p. 395) as C. subquadrata? are a little 

 different still; — the tops of the ribs are rounded or flattened and 

 are about as broad as the interspaces, near the umbones, or in 

 young specimens each rib is placed upon a low, broader rib some- 

 what after the nature of the costation in V. alticostata Con.; this 

 feature soon dies out however, leaving the ribs as Gabb has 

 remarked "of nearly the same shape as C. planicosta of the Eocene 

 formation." To us the most remarkable feature of Dr. Safford's 

 specimens is the paucity of ribs, numbering from 20 to 23 only. 

 The compound ribbing and the crenulation on the superimposed 

 ribs have been noticed in very young specimens of V. pla7iicosta 

 many times from different horizons and localities, but in these 

 specimens ■ it occasionally shows, though faintly, nearly to the 

 margin. Although there is no doubt as to the affinity of these 

 specimens, our collections from west Tennessee are too imperfect 

 to determine the geographical range or importance of this form, 

 hence we forbear from giving it a new varietal name. 



We see no special reason for confounding the variety smithi 

 with alticostata; the two seem quite distinct so far as our present 

 collections and knowledge go. 



It seems reasonable to suppose that when such a hardy animal 

 as V. planicosta had been introduced into terra nova as it was in 

 the Midway of our Southern States, it would, if the conditions 

 were favorable, multiplj^ rapidly, and as the sea changed or 

 alternated from a limestone forming, to a sand or clay depositing 



