4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
especially in the adult form. The New Mexico species is thus a more 
slender one, even the largest specimens never attaining the great 
thickness of C. compacta. 
The differences between the loops of the two species is very pro- 
nounced and is described below in connection with the loop of C. pro- 
lifica. (See text figure 1.) 
EXTERNAL MORPHOLOGY OF CRYPTACANTHIA PROLIFICA 
The abundant material on hand from the Magdalena limestone per- 
mits a detailed account of this species. Although the preservation is 
poor in many instances it is, nevertheless, possible to determine most 
of the features of the shell. 
Growth.—The smallest specimen (pl. 1, D, fig. 7) that could be 
identified with certainty as Cryptacanthia prolifica is paratype 
U.S.N.M. No. 127202j which is 1.1 mm. long, length of brachial valve 
0.87 mm., maximum width 0.94 mm., and thickness 0.44 mm. The 
specimen is oval in outline and has the maximum width at about the 
middle. The brachial valve is deeper than the pedicle valve, a condi- 
tion opposite to the adult, and the anterior commissure is uniplicate. 
The beak is straight, blunt, and has an open delthyrium. 
The pedicle valve of this specimen is nearly flat in lateral profile 
but with gentle curvature at the umbo. The anterior profile is broadly 
and gently convex. The umbonal and medial regions are gently 
swollen. 
The brachial valve of this small specimen is gently convex in 
lateral profile but with the posterior half more convex than the front 
half which is somewhat flattened. The anterior profile is broadly and 
moderately convex, most convex in the middle and with long sloping 
sides. The umbo is somewhat narrowly swollen. 
The smallest specimens are distinctly elongate-oval in outline and 
do not have the angularity so characteristic of the adult specimen. 
Sulcation of the anterior commissure starts at about the 2 mm. stage. 
At this stage, too, the shell begins to widen somewhat and develops 
some of the shouldered appearance of the adult. The young specimens 
remain distinctly elongate-oval until they reach about 6 mm. After 
6 mm., specimens appear that have length and width equal, but no 
specimen smaller than 5.9 mm. was measured with length and width 
equal. The larger specimens generally have length and width more 
nearly equal than those less than 6 mm. long. All the specimens meas- 
ured above 8 mm. have the length and width equal or the length 
slightly less than the width. 
