126 AMERICAN MARINE CONCHOLOGY. 



Subgenus CYRTOPLEURA, Tryon. 

 Monog. Plioladacea, 73. 18G3. 



Margin of the valves emarginate anteriorly, making a short, 

 wide hiatus. 



2. P. TRUNCATA, Say. Figs. 200, 2G1. 

 Journ. Philad. Acad., ii. 321. 1833. 

 Shell subpentaugular ; anterior obtusely rostrated, wedge-shaped 

 in the middle ; posterior margin broadly truncated at the tip ; 

 valves transversely wrinkled, crossed by stria?, muricated ante- 

 riorly by small erect scales, which form ribs from the beak to 

 basal margin. 



Yertical axis 1, transverse 2.5 inches. 

 Whole Coast from Sable Island to W. Indies; also W. Coast of S. A. 



Genus DACTYLINA, Gray. 

 Proc. Zool. Soc, 187. 1847. 



In the t3'pical form of this genus the nuclei of the dorsal valves 

 are situated at their outer margins, posterior to the centre; the 

 valves are much emarginate anteriorl}^, forming a short, wide 

 hiatus. I have characterized as a subgenus, a form of which our 

 American species is the tj'pe, as follov/s : — 



Subgenus GITOCENTRUM, Tryon. 

 Monog. Plioladacea, 75. 1863. 



Nuclei of the dorsal valves anterior, situated nearer the inner 

 margin ; dorsal plates marked by radiating lines ; valves not 

 emarginate anteriorl}', but regularly rounded ; hiatus long and 

 narrow. 



1. D. Campechensis, Gmelin. Figs. 2G2, 263. 



(Pholas.) Syst. Nat., 3316. 1790. 

 Pholas oblongata, Say, Journ. Philad. Acad., ii. 320. 1833. 

 Pholas Candeana, Orb. Moll. Cuba, 215, t. 25, f. 18, 19. 

 Pholas Chiloensis, King, Zool. Journ., v. 334. 1833. 



Shell thin, white transversely, much elongated ; basal and hinge 

 margins nearly parallel ; anterior and posterior margins rounded ; 

 valves transversely and longitudinally striated, the striie muri- 

 cated and elevated on the anterior side into costJE, which are 

 more prominently and densely muricated ; hinge callus polished. 



