1891.] 
343 
[Hyatt. 
distance of the posterior borders of these from the apex, as esti- 
mated, was about five inches and the whole length of the shell 
might have been more than twice that amount. These folds were, 
however, not accompanied by any ingrowth of the outer fibrous 
layer as in the specimen of S. missouriensis first referred to 
above. 
Similar but more pronounced sulci appear in the figure of the 
elongated form. Pinna quadrangular is sp. Goldf . 1 of the Creta- 
ceous. Pinna decussata Goldf. has a deep median, single, undi- 
vided sulcus in the cast of each valve. Goldf uss’s figures are so 
excellent and give the characteristics of thecarinae so accurately, 
that we can usually rely upon the details. If this be so, this last 
named species has characteristics in the adult precisely similar to 
what one ought to find in passage forms between Sulcatopinna 
and true Pinna. That is to say, its carina is masked by internal 
layers of nacre and the cast received in each valve an impression 
of a prolonged, unbroken, undivided median sulcus. This is simi- 
lar to the short depression occurring in the casts of Pinna, which 
is received from the internal nacreous keel in each valve but only 
occupies a limited space before the nacreous layer becomes di- 
vided. 
Pinna tetragona sp. Brocchi as figured by Hornes 2 has a deep 
median sulcation running the whole length of a long apical frag- 
ment, an undivided carina must have been present for a prolonged 
period in the growth of this shell. More information is required 
for the establishment of the fact that a modified form of carina 
may be present in the Carboniferous and I publish these remarks 
with the hope of being able to obtain suitable materials for 
further observations. 
The type of the genus Pinna established by Linnaeus is the well 
known form P. rudis Linn. The species has ridged shells with 
median carinae in the valves. The posterior muscular impressions 
are very narrow and not accompanied by auxiliary impressions. 
The nacreous layer is entire only near the apex of the shell, be- 
yond this posteriorly it is split in both valves under the carinae. 
The split widens at the posterior end forming a dorsal curvature 
around the muscular impressions and an opposite symmetrical 
iPetref. Germ., pi. 127. f. 8. 
2 Moll. d. Wien. Becken, Abh. geol. Reichsanst., IV. p. 347, pi, 51. 
