NO. 3 LOOP DEVELOPMENT OF CRYPTACANTHIA—COOPER 7 
In the 3.0 to 3.5 mm. stage the buttress plates unite on the floor of 
the posterior chamber and in the succeeding stage (3.6 to 4.0 mm.) 
gradually are raised as a distinct plate above the floor at the rear of 
the valve. A specimen 3.4 mm. long (U.S.N.M. No. 1272021) has a 
complete inner hinge plate with the anterior margin free but the 
posterior sessile and imperforate. After the 4.0 mm. stage the hinge 
plate consists of a single piece, commonly with a slight, or even pro- 
nounced, convex flexure but with a round or longitudinally oval 
foramen at the rear (pl. 1, E, fig. 22). A few specimens have no 
foramen but the adult generally is so provided. 
DEVELOPMENT OF THE LOOP: The collection here described does 
not include the complete development of the loop because no speci- 
mens smaller than 1 mm. were hollow and none of this size showed 
the loop. The earliest stages (0 to I mm.) must therefore be inferred. 
Fics. I-12 
All drawings by Lawrence B. Isham, scientific illustrator. 
ABBREVIATIONS 
di—descending lamella h—hood 
e —echmidium j—jugum 
f£ —foramen in hinge plate s—spines 
se—split in echmidium 
Figs. 1-10, Cryptacanthia prolifica Cooper, new species: 
1, Specimen in the 2.1-2.5 mm. stage showing slender lamellae and 
echmidium, X 10. (See pl. 2, B, fig. 3.) Margin restored. Paratype 
U.S.N.M. No. 127202s. 
Bue as above seen from the side and showing long point of echmidium, 
10. 
Specimen in the 3.1-3.5 mm. stage showing a stouter echmidium, > 10. 
Paratype U.S.N.M. No. 127203. 
Same as preceding but turned to side to show point of echmidium, X 10. 
Specimen in the 4.1-4.5 mm. stage showing a small but well-formed 
hood on the point of the echmidium, <8. (See pl. 2, F, fig. 10,) 
U.S.N.M. No. 127204h. 
6. Same specimen as preceding but turned to show elliptical opening of the 
early-formed hood, X 8. 
7. Same specimen as preceding, only slightly turned to the side but showing 
8 
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the small cuplike hood, X 8. (See pl. 2, F, fig. 11.) 
. Specimen in the 4.6-5.0 mm. stage showing fully formed and notched 
hood, 8. Paratype U.S.N.M. No. 127204t. (See pl. 1, I, fig: 27.) 
9. Same as preceding but loop seen partially from side, X 8. 
10. Specimen in the 3.1-3.5 mm. stage showing incipient hood on tip of 
echmidium, X 10. Paratype U.S.N.M. No. 1272031. (See pl. 2, D, fig. 7.) 
Figs. 11, 12, Cryptacanthia compacta: 
11. Dorsal view of the loop of Cryptacanthia compacta showing the long 
spines, deep cleft, jugum, and short descending lamellae. Dotted lines 
indicate where part of loop was dissolved away but jugum is intact. 
(Compare pl. 1, A, fig. 1.) Hypotype U.S.N.M. No. 9382a. 
12, Same as preceding but showing loop from side with its long spines, 
prominent erect hood, and long descending lamellae. (Contrast with 
profile of loop of C. prolifica, pl. 2, H, fig. 16.) 
