- 37 - 



Aroa um"boriata lamarck, Dall, 1915, Bull. U. 3. Fat. Mus., no. 

 90, p. 118, pi. 17, figs. 6, 8. 



Desoriptior- .- "A. testa transversim o^blonga, ventrieosa, 

 angnJato-sinuata, deciissatim substriata; "um"bonilDus taagnis, arc- 

 iiatis; Istere postioo -orevissimo." (lamarcic, 1055) 



The Bowden spesir^.ens are small, subrhomboidal , much inflate^, 

 very inequilateral; anterior margin almost at right angles to the 

 long, straight dorsal mar?-in, short, curving below into the ventral 

 margin, v.'hicfc is v er j- slightly arouate and feebly emarginated ant- 

 erior to the middle by the small byssal ga-'e; obliquely truncated 

 posteriorly; urabcnes very tumid, prominent, strongly incurved, o- 

 pisthogyrate, placed at about the anterior third of the length of 

 the hinge; posterior keel strong, but not excessively sharp or 

 high; region dorsal to the keel slightly excavated,-; exterior sur- 

 face anterior to the posterior keel covered m th radial ribs, rel- 

 atively coarse except in the region of the byssal gape; in the 

 anterior half of the shell one or tv/o fine radiating ribs often 

 present in the interstices; the radial ribs crossed by weaker, low, 

 irregulrr concentric ridges, producing a subimbricated sc;lpture; 

 dorsal to the posterior keel three or four coarse, subnodulous 

 ribs, succeeded by several finer obes; cardinal area very wide^ 

 the anterior portion nar'red by several oblique ligament grooves; 



