1847.] SHARPE ON THE PALZOZOIC ROCKS OF N. AMERICA. 175 
pear on the worn outside of the shell like a single plate ; while in the 
true P. galeatus two plates are seen on the worn surface of the shell 
forming a fork at the beak. 
The small Eifel species has not yet received a name, having been 
confounded with the P. galeatus. M. de Verneuil has unfortunately 
taken an Eifel specimen, with the plates of the ventral valve united, 
as the type of the Silurian species*, although the Russian specimen 
figured at pl. 8. f. 3 c. shows the two ventral plates quite distinct ; 
and from this circumstance he has confounded together the characters 
of the two species in describing P. galeatus. 
Both species here alluded to are found in the Eifel, but only the 
true P. galeatus has been found in the Silurian rocks of Europe or 
the United States. 
(10) Sprrirer BIroratus.—Few shells present so many varia- 
tions of form as this elegant species ; yet there are certain characters 
common to all its varieties which give them a marked general re- 
semblance, and should prevent our subdividing the species, or con- 
founding it with any other. The principal variations are well-de- 
scribed, and its more important characters shown in Mr. Hall’s 
‘ Paleontology,’ p. 132, &c. 
Mr. Hall has pomted out that in the young state there are in- 
variably two plaits on the mesial fold of the ventral valve, and one 
plait raised in the middle of the deep sinus of the dorsal valve. I 
can confirm the accuracy of this remark after examining above a 
hundred specimens. Each of these plaits may either continue simple, 
or bifurcate once or twice, forming the three principal varieties which 
have been described. 1st, where the plaits are simple, the shell is the 
Spirifer chama, Lichwald, with one plait in the smus, and two plaits 
on the mesial fold. This is a rare form of the full-grown shell in the 
United States. 
2. Spirifer lynx has each of the two original plaits on the mesial 
lobe increased to two by bifurcation, making four plaits im all; while 
to correspond with these, the original plait in the smus has branched 
into three. This is the common American variety, which varies 
almost indefinitely in the number of the side plaits and in the length 
of the hinge-line. The most remarkable variety is that which Con- 
rad has named. Delthyris acutiliratat, which has sometimes twenty 
plaits on each wing. 
3. Spirifer biforatus, having six plaits on the mesial fold and five 
in the sinus, arising from a double bifurcation of the original plaits, 
This form is only seen in very old shells, and is rare everywhere. 
The side plaits, although varymg in number from four to twenty, 
are usually simple: if a bifurcation is seen it is a rare accident, pro- 
bably arising from injury to the shell during its growth, and is as 
common in the forms with few as with many ribs. It sometimes 
happens that one of the plaits on the mesial fold bifurcates while the 
other remains entire, or that one bifurcates once and the other twice, 
* Russie, vol. ii. p. 121. 
+ Journ. Acad. Nat. Sciences of Phil. vol. viii. t. 14. f. 15. 
