INVERTEBRATE PALAEONTOLOGY. 259 



distinction ; though it is not always easy to determine whether or not fossil 

 species may have had such supplementary parts. 



The geological range of this genus has not been very satisfactorily deter- 

 mined, but we have some reasons for believing that it may possibly date 

 back to the Permian or Carboniferous period. At any rate, a shell described 

 by the writer from rocks apparently of Carboniferous age in Texas (Martesia ? 

 Roessleri) was thought, by Mr. Tryon, of Philadelphia, an excellent authority on 

 the Plioladacea, most probably to belong to this genus. In the Jurassic rocks, 

 its existence seems to be established by the discovery of several species, 

 among which may be mentioned M. recondita (= Pholas recondita, Phillips, 

 Geol. Yorks., pi. 3, tig. 19). Several well-defined species of this genus are 

 also known from the Cretaceous, and it also ranges through the Tertiary, 

 and is likewise represented in our existing seas, where it probably about 

 attains its greatest development. The living species are generally found 

 burrowing in floating wood, and seem not to be entirely marine in their 

 habits, as H. and A. Adams mention finding M. rivicolor perforating floating 

 logs in t lie river Pantai, in Borneo, twelve miles from the sea, where the 

 water was perfectly fresh. 



Martesia cuneata, M. &H. 



Plate 30, figs. 8, a, b. 



Pholas cuneata, Meek and Hayden (March, 1858), Proceed. Acad. Nat. Sci. Philad., X, 53. 

 Fhulas (Martesia) cuneata, Meek and Hayden (Oct., 1860), ib., XII, 424. 



Shell small, thin, transversely elongated, and a little arcuate ; gibbous 

 and truncated anteriorly, cuneate, and very sharply rounded behind; posterior 

 gape very slight ; anterior hiatus formed by a rectangular notch extending 

 about half-way up from the beaks, and but a short distance back from the 

 anterior margin ; callous plate filling the hiatus at maturity, very thin, smooth, 

 and slightly ventricose ; beaks depressed, incurved, and nearly terminal ; sur- 

 face marked by fine lines of growth, and small, rather obscure, concentric 

 costae or wrinkles, which are only visible on the more gibbous anterior and 

 antero-central portions of the valves; umbonal furrow linear, extending 

 a little obliquely backward and downward, so as to intersect the base in 

 advance of the middle. Accessory valves unknown. 



Length, 0.37 inch; height at the anterior end, 0.18 inch: convexity, 

 0.16 inch. 



