BRADLEY: lUtAeilini'ODA of THE MAQUOKETA OF [OWA. 525 



were more convex than their diameter in any other direction. The 

 posterior lateral margins meet the beak sharply in young shells, bul 



become more rounded in the adult. 



The brachial-valve is always a little more convex than the pedicle 

 and marked by a mesial ridge nearly flat and topped with four plica- 

 tions. This continues about two thirds the way to the beak which 

 is strongly incurved; lateral slopes bear from four to eight simple 

 plications. 



The pedicle-valve terminates abruptly in a pointed beak which 

 becomes strongly incurved upon that of the other shell in adults. 

 The mesial sinus is deep and well defined in mature examples, hearing 

 three or four sharp plications; lateral slopes each have five to seven 

 simple striatums. 



The entire surface of both valves is marked by numerous very 

 regular, strongly zigzag, and prominent lines of growth. 



Rhynehotrema capax is larger than R. increbescens with which it is 

 sometimes confused and has a greater convexity and thicker valves. 



This species varies much in the development from the young to the 

 adult form and has caused the name of R. capax to cover a multitude 

 of faulty identifications. 



Locality: — Richmond at Richmond, Indiana; Oxford, etc., Ohio; 

 Illinois; Iowa; Tennessee; Missouri; Wisconsin; Minnesota; 

 Anticosti; Lake Winnipeg, Manitoba; Fort Churchill, Hudson Bay, 

 etc. 



Zygospira tantilla, sp. nov. 



Shell small, biconvex, the pedicle-valve sharply keeled in the 

 umbonal region, the anterior edge sharp. The brachial-valve is 

 convex at the umbo, becomes flattened in front, with a broad shallow 

 median sinus. The surface is covered with fine plications, the median 

 one on the brachial-valve slightly thicker than the others. On either 

 side of the median plane there are ten to twelve ribs. 



Measurements: — The largest specimen is 5.75 mm. long and 5.50 

 mm. wide. A small one is 4 mm. long and 3 mm. wide. 



This species is very like Z. rccurvirostris, and differs from the other 

 Richmondian forms in its small size. It is a less robust shell than 

 Z. recurmrostris, and has fewer plications than Z. modesta. It is not 

 unlike Z. kentuckiensis, but is smaller, less plump, and that species 

 does not appear to show an enlarged median plication. 



Locality: — Twelve specimens were collected from the Lower 

 Maquoketa near Clermont, Iowa. Holotye, M. C. Z. 8,547. 



