198 AMERICAN JOURNAL 



MESALIA, Gray. 



M. Ortoni, Gabb.— PL 16, fig. 3. 



Description. — Shell small, elongated, spire high, whorls eight 

 or nine, sometimes nearly plain, or in other cases marked by two' 

 or more large revolving carinas in the young state, which always 

 disappear as the shell grows older ; the larger whorls are smooth, 

 flattened on the sides and round in above and below 7 , to the su- 

 ture, which is deeply impressed ; base of body whorl rounded. 

 Aperture subovate, acute behind, rounded in advance ; outer lip 

 thin and straight, inner lip acute and slightly reflected over the 

 umbilical region. 



Dimensions. — Length, -35 in., width, -13 in. 



TELLINA, Linn. 

 T. Amazonensis, Gabb. — PI. 16, fig. 4. 



Description. — Shell small, thin, elongate subquaclrate, slightly 

 tapering posteriorly ; beaks about a fourth of the length from 

 the anterior end; cardinal margin very slightly arched and 

 sloping; base broadly convex, most prominent near the middle; 

 anterior end produced and regularly rounded ; posterior end 

 rounded and a little narrower than the anterior ; surface marked 

 only by fine lines of growth ; hinge delicate; teeth very minute. 



Dimensions of a very small specimen. — Length, *25 in., width, 

 •15 in., height of single valve, -04 in. Fragments of another 

 specimen were found among the debris, twice as long as the pre- 

 sent shell. 



PACHYDON, Gabb. N. Gen. 



Shell oblique, inequivalve, very inequilateral ; the right valve 

 a third or more deeper than the left ; ligament very small, ex- 

 ternal. Hinge composed of an enormous oblique tooth in the 

 right valve ; and in the left, of a corresponding pit and a linear 

 tooth articulating above that of the right ; there is also a rudi- 

 mentary posterior lateral. Muscular scars large ; pallial line 

 slightly sinuated. Surface smooth, or marked only by lines of 

 growth ; internal margin entire. 



The general appearance, the prominent tooth, the slightly sin- 

 uated pallial margin and the unequal valves of this shell would 

 seem to point to the Corbulldce for the relations of this peculiar 

 little genus. It has, however, no trace of a cartilage insertion. 

 On comparing it with Isocardia, several points of resemblance 

 occur, which incline me to associate it with that genus, as a mem- 

 ber of the lsocardiidce, despite its inequivalve form. It resem- 





