8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



valve middle; plates bounding sockets broad; outer socket ridges 

 broad ; sockets long and deep. Crura short ; crural processes short and 

 pointed ; loop having form of early dallinid development, braced at its 

 junction with the septum by a wide plate concave toward the anterior ; 

 descending branches slender; ascending branches broad; transverse 

 ribbon broad and with two prongs on each side which are directed 

 toward the pedicle valve. Hinge plate usually obscured by callus which 

 smooths the notothyrial region ; cardinal process a wide, bilobed thick- 

 ening on the posterior margin of the notothyrial platform formed by 

 the callus covering the hinge plate. 



Development of the loop. — The smallest specimens showing interior 

 details are 2.0 mm. wide (paratype U.S.N.M. No. 124200a) and 2.7 

 mm. (paratype U.S.N.M, 124198a). In these specimens the septum 

 is continuous from the hinge platform to the valve middle where it is 

 expanded ventrally toward the pedicle valve to form the pillar. Viewed 

 from the anterior the expanded pillar is divided by a groove and the 

 ventral and dorsal sides diverge slightly from each other, the begin- 

 nings of the loop ring. Crural processes and descending branches are 

 either not preserved or not yet formed, probably the former. The 

 notothyrial region is filled with solid callus. 



The next larger specimen is 3.6 mm. in width (paratype U.S.N.M. 

 No. 124199). Both valves are preserved. The pedicle valve shows a 

 wide and completely open delthyrium with no trace of the deltidial 

 plates. Thickening on the floor of the delthyrial cavity is strong and 

 the median ridge anterior to this thickening is well developed. Inside 

 the brachial valve the notothyrial region is solid as in the preceding 

 specimen, but the septum is short and high. The crural processes are 

 well developed and the crura are short and thick. The descending 

 lamellae join the lower or dorsad diverging branches of the anterior 

 end of the pillar, the anterodorsal side of which is now more deeply 

 cleft. The ventral portion of the septum is elongated and the two 

 lamellae diverging widely from the pillar are roofed by a flat plate, 

 the pre-campagiform hood of Elliott. 



A third specimen of 5 mm. width (paratype U.S.N.M. No. 124201a) 

 also shows the median septum and its anterior expansion. This speci- 

 men differs from the preceding one in having a definite concavity in 

 the notothyrial callus, bounded by the crural bases. Other details of 

 the loop can only be inferred. 



A fourth specimen 5.7 mm. in width (paratype U.S.N.M. No. 

 124198b) shows additional details of the loop, but part must be in- 

 ferred from ridges and remnants. The notothyrial callosity is thick 

 and is buttressed by a strong median septum. The crura are very short 



