49 American East Coast Arcas 49 



in crossing the ribs develop into sharp, thick transverse nodulations; cardinal area very 

 narrow and with an elevated margin behind, slightly wider in front of the beaks longi- 

 tudinally striate; ends of the hinge-line angular; anterior end bluntly rounded, base par- 

 allel with the hinge-line, posterior end subtruncate, a little produced below; hinge with nu- 

 merous rather crowded subvertical teeth in an uninterrupted series; inner margin of the 

 valves deeply fluted. Lon. of largest valve 28, alt. 14; of younger valve 18, alt. 8.5, 

 diam. 7 mm. 



"The larger valves of this rare species are distorted or worn so that a younger one 

 has been selected for figuring. The most conspicuous feature of the shell is the deep sul- 

 cation of the beaks, which gives them a bilobed appearance."- — Dall, 1898. 



Ocairrcnce. — Pliocene marls of the Caloosahatchie River, Florida. — Dall. 



Gabb described Aira Cliiriquietisis from the Tertiary of Chiriqui, Central America 

 (Acad. Nat. Sci. Phila.,Proc. for i860, p. 567, 1861). Later, (Acad. Nat. Sci.Phila., Journ.,2d. 

 sen, vol. 8, pp. 345 and 378), he placed this species, together with fossils from St. Dom- 

 ingo and Costa Rica and Sowerby's A. patricia (Quart. Journ. Geol. Soc. Lond., vol. 6, 

 p. 52, 1850) from St. Domingo, all under A. grandis Broderip and Sowerby, (Zool. 

 Journ., vol. 4, p. 365; Reeve, Conch. Icon., Area no. 4, 1843). Dall, (Wagner Free 

 Inst. Sci., Trans., vol. 3, p. 642), calls the Chiriqui and St. Domingo fossils Oligocene 

 and separates them from grandis under the name Scapkarca {Scapharca) chiriquiensis 

 Gabb. He says that the species ' 'has about thirty rounded ribs with subequal channelled 

 interspaces, the anterior ribs being granulose or nodiferous, the shell remarkably high, 

 short, solid, and wide. The measurements of a well-grown specimen are: alt. 42, lon. 

 45, and diam. 44 mm.; the length of the cardinal area is 28 mm. It is one of the spe- 

 cies on the border line between Scapharca and Auadara, the two valves being similarly 

 sculptured and almost equal." 



Evidently at least two species of fossil shells have been confused here. In the Gabb 

 collection from St. Domingo there are two valves as large and heavy as specimens of A. 

 grandis from the west coast. These fossils do not show enough variation from the re- 

 cent form to warrant placing them in a separate species. There are a few small valves, 

 the largest 23 mm. long, of a short, well-sculptured Scapharca which are distinct from 

 the young of A. gra?idis. They are short and high like A. chiriquiensis, but unlike this, 

 the left valve is more nodulous than the right, conspicuous nodules extending over nearly 

 the entire valve, and the ribs of the left valve are square and wider than the interspaces. 

 There are also from St. Domingo two worn valves which resemble the yovmg of A. 

 grandis in form but appear to be a separate species. The lack of figures of A. patricia 

 and A. chiriquiensis has added to the confusion. 



Scapharca {Scapharca) halidonata Dall; Plate XI, Figure 8; (Wagner Free Inst. Sci., 

 Trans., vol. 3, p. 646, pi. 33, fig. 24, i8g8), from the Bowden beds, Jamaica, and of Cu- 

 rasao is of the general type of A. secticostata but is proportionately mtich shorter. The 

 fossil .-4. consobrina Sowerby; Plate XI, Figures 9, 10; (Quart. Journ. Geol. Soc. Lond., 

 vol. 6, p. 52, pi. 10, fig. 12, 1850), from St. Domingo also belongs to the group of which 

 A. secticostata is the recent representative. The name consobrina had already been used 



