102 



THE NAUTILUS. 



radial costae ; near the deep radial sulcus (lie 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 stri*. 

 Length, H-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 DIPLOTHYRA Try on, 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 caribata Orb., Historia, etc., p. 281, pi. 25, f. 20-21, 1845. 

 French edition, p. 211, t. 25, f. 20-21, 1853. 



Diplothyra Smithii Tryon, Proc. Acad. Nat. Sci. 1862, p. 450; 

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



Shell broadly wedge-shaped, inflated anteriorly and tapering ab- 

 ruptly towards the posterior; the anterior half with fine wavy lines 

 forming slight radial costoe, radial sulcus (yuite prominent, posterior 

 half marked only by small concentric undulations and growth lines; 



