8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
Stages 1.1 to 1.5 mm.: The smallest specimen (U.S.N.M. No. 
127202m) showing traces of the loop is a brachial valve 1.25 mm. 
long and had a pedicle valve estimated to be 1.50 mm. long. In this 
specimen only the descending lamella on one side is visible as a thin 
and delicate ribbon with an angular bend medially at about midvalve 
(0.4 mm. anterior to the brachial beak). The fact that the descending 
branch bends medially suggests that at this stage the anterior ends 
of the loop were united. It is probable that in stages below 1.0 mm. 
only the initial stages of the descending lamellae are present and are 
not bent medially at their distal ends. 
Stages 1.6 to 2.0 mm.: Two specimens (U.S.N.M. Nos. 127202k, 
127202n) measuring between 1.5 and 2.0 mm. in length exhibit the 
loop. Specimen U.S.N.M. No. 127202n is exactly 2.0 mm. long and 
1.68 mm. wide. The entire loop is not visible but the pointed anterior 
can be seen and indicates a loop of about 0.8 mm. in length. The loop 
of the other specimen (U.S.N.M. No. 127202k) is broken on one side 
(pl. 2, A, fig. 1) but the descending branches are thin and delicate 
and bent medially about 0.4 mm. anterior to the beak. These two 
specimens thus indicate a loop of about 0.8 mm. in length, the lateral 
branches of which bend medially and unite distally in a sharp point. 
Stages 2.1 to 2.5 mm.: Information on these stages is based on 5 
specimens (U.S.N.M. Nos. 127202-0, p, q, r,s). Three of the speci- 
mens are 2.4 mm. long and 2.0 mm. wide, but two are 2.3 mm. long 
and 2.0 mm, wide. The loop varies in length from I.0 to 1.2 mm. 
The loop is much stouter than in the preceding stage and the crural 
processes are visible as blunt points. The angular bend of the de- 
scending lamellae is present and the two branches unite distally to 
form a sharp, angular, pointed plate not greatly extended anteriorly. 
The line of junction between the descending branches varies from 
0.3 to 0.4 mm. The descending lamellae thus unite to form a spear- 
shaped plate here called the echmidium.t This name is introduced 
because, in the development of the loop, the spear-shaped plate be- 
comes increasingly prominent and ultimately is the site on which grow 
the ascending elements of the loop. At the 2.1 to 2.5 mm. stages no 
ascending elements or their beginnings were seen in any of the 
specimens. 
Stages 2.6 to 3.0 mm.: Thirteen specimens with this range, hav- 
ing the loops preserved, appear in the collection (U.S.N.M. Nos. 
127202t-z, 127203a-f). The length of the loop averages 1.33 mm. and 
1 Echmidium is derived from aichmidion, diminutive of aichme, point of a spear. 
The ai is transliterated e. 
