MOLLUSCA. 653 



diameter of 9 mm. The dimensions of a large shell are : length, 

 10 mm. ; height, 9 mm. ; depth of one valve, 5.5 mm. The hinge- 

 line is about .7 of the total length of the shell. In anterior view 

 the shell is cordate in outline, each valve being irregularly sub- 

 ovate in lateral view. Beaks in front of the middle of the shell, 

 pointed and strongly incurved, umbones very prominent, the 

 valves compressed posteriorly and not gaping. The antero-basal 

 hiatus rectangular, large and deep, occupying nearly the entire 

 anterior side of the shell, its upper margin two-thirds the total 

 height of the shell from the ventral margin. Ventral and pos- 

 terior margins rounded. Umbonal sulcus deep and narrow and 

 slightly oblique, on each side the surface of the valve is raised in 

 a slight rib which becomes stronger, especially the posterior one, 

 towards the ventral margin of the valve. In the internal cast 

 a shallow ill-defined furrow originates on the posterior side of 

 the beak and extends obliquely backward towards the postero- 

 ventral margin, becoming almost obsolete as it approaches the 

 margin. On the anterior side of the beak a narrower and shal- 

 lower, but more sharply defined furrow originates and continues 

 to the inner angle of the anterior hiatus of the valve. The finer 

 surface markings are not well shown upon the internal cast, but 

 they apparently consist of fine and inconspicuous lines of growth. 



Remarks. This species is a member of Meek's subgenus 

 Goniochasma. The type specimens are from the same fragment 

 of fossil wood which has furnished the examples of Martesia 

 cretacea. The largest one has occupied a burrow, now filled with 

 sand and pyrite, 9 mm. in diameter, whose direction is with the 

 grain of the wood nearly in the center of the specimen. The 

 tubes of this species are essentially identical with those of Teredo 

 irregularis, and without knowledge of the shells themselves the 

 two forms cannot be distinguished. The burrows of Martesia cre- 

 tacea, however, are all normal to the surface of the wood which 

 they penetrate only a short distance deeper than the length of 

 the shells. 



A single example of the species has been observed from the 

 Merchantville formation at Lenola. This specimen is a smaller 

 one, only 6.5 mm. in length, and is a very imperfect internal cast. 

 So far as can be determined it has essentially the same form and 



