1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 101 



valve, short, ^-shaped, received iu the right valve into a corresponding 

 sulcus, below which a triangular pustule represents the anterior lateral 

 of that valve; cardinal teeth of the left valve diverging, lamellar, the 

 anterior lamella situated above the anterior lateral tooth, fitting above 

 a triangular cardinal tooth grooved or partly split at the apex, in the 

 right valve; posterior lateral tooth in the left valve long, arched, finely 

 crenate above, received in the right valve between two sleud«n- lamina*, 

 of which the lower one is most ])rominent; pit for the ligament and 

 cartilage narrow, oblique, roofed over by a very thin shelly layer gen- 

 erally worn off in rubbed specimens. Length of shell 17.5 ; height 1.15; 

 double diameter of valve, 10 mm. Fragments indicate that the species 

 reaches a length of at least 25 mm. 



This species differs from the young of G. euneatii.s in the fact that the 

 ^-shaped cardinal tooth is in the right valve when the valves are closed, 

 while in G. cuneatns it is in the left valve, as well as in clathrodon, Lecon- 

 tei, mendicus and.//e.r»o.s^^v. In G.Johnmui the anterior lateral tooth is 

 shorter, relatively, than in any other species, and the shell is more 

 drawn out behind the beaks. 



The geological age of this species is somewhat in doubt. It is asso- 

 ciated with Hydrohia mohiUana Dall, and with a large oyster and MuUnia 

 lateralis var. eorhuloldc.s Eeeve. The latter is a living species and is 

 not otherwise known below the newer zones of the Chesapeake Miocene. 

 The Pascagoula clays were referred to the Grand Gulf beds by Hilgard, 

 and overlie them. There is no doubt that the typical Grand Gulf beds 

 are included between the Hawthorne beds, at the base of the older Mio- 

 cene, and certain beds of the Chipola series; at present it seems im- 

 probable that the Pascagoula clays can be correlated with anything older 

 than the Chesapeake. I am disposed to consider them as correspond- 

 ing to the aluminous clay above the Chesapeake clay-marl in the Alum 

 Bluff series. 



.Subgenus Kangianella, C our ad. 



RangianeUa, Conrad, Am. Journ. Conch., iii, Snppl. p. :^(», 18(57. 

 liangia, Carpenteu, Mazatlan Shells, p. 53, 1857. 



Lateral teeth short, subequal, about equidistant from tbe beaks, 

 feebly striated or smooth; shell ot moderate size, subelongate or ros- 

 trate, longer than high; pallial sinus inconspicuous or obsolete. 



Type: Gnathodou fri<foni(,s, Petit=6^. mendicus., Gould. 



The hook of the anterior lateral tooth is almost obsolete in this 

 species, especially in the young, and it was chiefly upon this character 

 that Conrad separ^ed it, leaving G. Jlexuosus with the typical species; 

 but the sum of all the characters, if taken into account, Avould modify 

 this view. Carpenter saw the difference ten years earlier, and would 

 have utilized the name i?aw</t« for the short-toothed species; but this 

 proceeding would be contrary to the accepted rules of nomenclature, 

 since Rangia was based solely upon the same si)ecies as Gnathodon, and 

 must stand or fall with the ])riority of application to that special type. 



