102 



THK NAUTILUS. 



radial costae ; near the deep radial sulcus tlie crenulations are want- 

 ing, and beyond the sulcus are merely concentric undulations or 

 growth lines, callum smooth, line of attachment rounded, cordate ; 

 protoplax arrow-shaped with a medial depression and oblique strite. 

 Length, 14-18. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



Connecticut to the West Indies. Near New Haven, Ct., in oyster 

 shells (Perkins); Holly Beach, N. J. (Ford); Oceanus, Fla. (White), 

 and found by the writer at St. Augustine. 



Subgenus Diplotiiyra Tryon, 1862. 

 The protoplax and also the metaplax are bordered by an elevated 

 callous margin ; in the former case obliterating the deep depression in 

 front of the umbones. Metaplax and hypoplax divaricating. Tryon 

 considered the sculptured and smooth portions of the protoplax as a 

 "double accessory valve," and on that character founded the genus 

 Diplothyra. The above characters seem to separate it subgenerically 

 from the typical Martesia. 



Martesia (Diplothyra) carib.ea (Orbigny). Fig. 3. 



Pholas caribcea Orb., Historia, etc., p. 281, pi. 25, f. 20-21, 1845. 

 French edition, p. 211, t. 25, f. 20-21, 18o3. 



Diplothyia Smithii Tryon, Proe. Acad. Nat. Sci. 1862, p. 450; 

 Mon. Pholadacea, etc., p. 126, pi. — , f. 2, 1862. 



Shell broadly wedge-shaped, inflated anteriorly and ta[)eriiig ab- 

 ruj)tly towards the posterior; the anterior half with fine wavy lines 

 forming slight radial costae, radial sulcus quite prominent, posterior 

 half marked only by small concentric undulations and grovvtii lines; 



