Wetter — Kinderhook Faunal Studies. 173 



posterior (short) end, prominent and incurved. Hinge-plate 

 bent abruptly beneath the beaks ; occupied by from five to 

 seven long narrow teeth on the long side and from three to 

 five smaller ones on the short side. Posterior end broadly 

 rounded ; anterior end prolonged, obtusely pointed ; basal 

 margin strongly arcuate, and the border of the shell thick- 

 ened." 



" Surface characters not determined. This species, like 

 most of the others, occurs in the condition of internal casts, 

 and in some instances the impressions of the exterior surface 

 have not been preserved." 



" This shell appears to be subject to considerable variation, 

 at different stations of growth ; young specimens often being 

 distinctly triangular, with the posterior end very short, and 

 the basal margin but little arched, while old specimens are 

 subovate in form and the posterior end more prolonged. In 

 one full-grown individual the muscular impressions are very 

 strongly marked, the anterior one being nearly double the 

 size of the posterior, and the basal portion of the shell shows 

 a great degree of thickening." 



The dimensions of a rather large specimen are: length 13 

 mm., breadth 10 mm., and convexity of one valve 4 mm. 



Pal aeon eilo microdonta (Win.). 



PI. XV. f 15-16. 



Nucula microdonta, Bull. U. S. G. S. 153:387. 



Original description. " Shell small, transversely oblong; 

 height equal to two-thirds the length ; beaks small, somewhat 

 incurved, but little elevated above the hinge-line, about one- 

 third the length from the short end. Ventral border rapidly 

 curved, and regularly so to the vicinity of the long end, 

 where it is slightly sinuated, from which point a shallow 

 groove extends up nearly to the beak. Dental plates but little 

 angulated between the beaks; the larger bearing near its 

 outer margin 10 or 12 minute transversely tubercular teeth, 

 and the shorter 4 or 5. Teeth not distinguishable to the 

 beaks, but no cartilage pit seems to be present. Anterior 

 muscular pit oblong, surrounded by a large pedal scar. Shell 

 most ventricose in the middle. No surface markings dis- 

 cernible." 



