ARCONAIA I 187 



Diplodon ludoviciamim RochebrunK, Bull. Soc. Phil, 1882, 



p. 43. — Simpson, Syn., 1900, p. 862. 

 Trapesoideus ludozncianus Haas, Conch. Cab. Uiiio, 1912, pi. 



32. fig. 5- 



This species, which was placed among the indeterminate 

 species in the Synopsis, has recently been figured as a Trap- 

 eaoideiis by Haas. (1. c), but the accompanying text has not 

 yet been published. 



Genus ARCONAIA Conrad, 1865. 



Arconaia Conrad, Am. Jl. Conch., I, 1865, p. 234. 



Shell greatly elongated, inflated, solid, twisted on its axis, 

 sometimes straight, but generally having the posterior end 

 curved strongly to the right or left, with a decided posterior 

 ridge on both sides of the shell, ending in a blunt point, with 

 generally a small ridge above on the post-slope ; beaks rather 

 low, sculpture not seen, but probably zigzag-radial ; often a 

 few nodulous or wavy ridges cross the body of the shell, 

 which is irregularly sulcate ; epidermis rayless; two pseudo- 

 cardinals in the right valve, the upper compressed, separated 

 from the split-up lower one by a parallel-sided pit, two in the 

 left, and two heavy laterals; one lateral in the right and a 

 vestige of one below it, laterals granulated and vertically 

 striate; muscle scars deep, anterior small, the front and upper 

 united, the hinder separate ; posterior muscle scars long, 

 oblique ; beak cavities very shallow ; dorsal scars numerous, 

 placed just on the inner edge of the hinge ; nacre white. The 

 lobes of the mantle are not united into siphons, but separated 

 throughout, with only a feeble commissure separating the anal 

 and branchial openings. 



Type, Triquctra lanceolata Lea. 



In 1865 Conrad established the genus Arconaia for the Tri- 

 quetra lanceolata of Lea. T have placed in it. besides the 

 original form, two others wiiich Heude doubtfully believes to 

 be varieties of lanceolata, but which seem to me to be valid 

 species. The systematic position of the group is a little uncer- 

 tain, but I am inclined to believe that it is one of the Endo- 

 branchs and should be placed in the supergeneric group 

 Rosan rh aniph us. 



