MIOCENE MOLLUSCA AND CRUSTACEA. 47 
ARCA (LATIARCA?) IDONEA?, 
Plate vu, fig. 1. 
Arca idonea Conrad: Foss. Shells of the Tert. Form., p. 15, Pl. 1, fig. 5; Miocene 
Foss., p. 55, Pl. xx1Xx, fig. 3; Emmons, Geol. N. Carolina, 1858, p. 285. 
Latiarca idonea Conrad: Proc. Acad. Nat. Sci., Phil., 1862, p. 289. 
Arca idonea? Heilprin: Proc. Acad. Nat. Sci., Phil., 1887, pp. 400 and 402. 
Scapharca idonea Conrad: Proc. Acad. Nat Sci., Phil., 1862, p.579; Meek, Check List 
Miocene Foss., p. 6. 
Arca stillicidium Conrad: Foss. Shells Tert. Form., Pl. 1, fig. 3; Miocene Foss., p. 55. 
“Shell subtrigonal, thick, diameter equal to about seven-eighths of the 
length; ribs about thirty, flattened on the back and angulated on the sides, 
those on the anterior side with a longitudinal furrow; ribs of the right valve 
crenulated over the whole disk; of the left valve distinctly crenulated only 
on the anterior side; crenulations largest on the right valve; beaks distant 
and very prominent; cardinal line short, a little decurved at the ends; teeth 
irregular and some of them angulated in the middle; inner margin pro- 
foundly crenate.” (Conrad, Mioc. Foss., p. 55.) 
Two small fragments of shell, presenting much more the characters of a 
Cardium than of an Arca, come to me among the material from the well- 
boring at Atlantic City, and are the foundation for the citation of the above 
species made in the Proceedings of the Academy of Natural Sciences, Phila- 
delphia, 1887, pp. 400 and 402. The distance of the ribs from each other is 
equal to their own diameter and they are rounded on the top, more so than 
in Arca idonea, from the amount of sand wearing which these fragments 
show, while one of them shows a nodose character of the ribs more charac- 
teristic of a Cardium than of an Arca. I may be wrong in my suspicions 
that they are not fragments of the species to which they are referred, for it 
is very difficult to assert positively the relations of such small fragments of 
water-worn shells. 
The specimens are the property of the Academy of Natural Sciences 
at Philadelphia, Pa. 
