256 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



with any of the numerous specimens found in their burrows; while none of 

 them show any indications of the distinctly rectangular anterior hiatus having 

 ever been closed by a shelly plate, at any stages of their growth. In addi- 

 tion to these peculiarities, internal casts always show a broad, oblique furrow, 

 just above each posterior umbonal slope, (see fig. 9 a, pi. 30), that must have 

 been occupied by a corresponding posterior internal ridge in each valve, not 

 found in Martesia, and apparently representing the narrow, less oblique, pos- 

 terior internal ridge seen in each valve of Turnus. 



Mr. Grabb does not say whether his typical species of Turnus is gaping 

 posteriorly or not, though Dr. Stoliczka describes the Indian species pub- 

 lished by him as being nearly closed behind. He expresses the opinion, 

 however, that, as the animal secreted a shelly tube, the valves " must have 

 been much more gaping posteriorly than they are in the recent Xylophagtz, 

 which only slightly protrude out of their shells." Large shelly tubes, some- 

 times an inch in diameter, and eight or nine inches in length, were also found 

 so associated with the California typical species of Turnus, as to lead to the 

 conclusion that they were secreted by the same. So far as yet known, how- 

 ever, no shelly tubes have been found associated with either of the Upper 

 Missouri species, although large numbers of these shells have been found 

 penetrating masses of fossil wood. Both of the Upper Missouri species also 

 seem to have the valves always tightly closed behind. 



So far as has been positively determined, these shells occur only in the 



Upper Cretaceous; though, as remarked by Stoliczka, Pholas Waldheimii, 



d'Orbigny, from the Jurassic or Neocomian of Russia, probably belongs to the 



group. 



Turnus (Goniocliasma) Stimpsoni, M. & H. 



Plate 30, figs. 9, a, b. 



Xijlophaga Stimpsoni, Meek and Hayden (May, 1857), Proceed. Acad. Nat. Sci. Philad., IX, 141. 



Pholas? Stimpsoni, Meek and Hayden (Oct., 18G0), ib., XII, 424. 



Goniocliasma Stimpsoni, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 1G. 



Shell transversely ovate-cuneiform, very thin; anterior and umbonal 

 regions gibbous, more compressed posteriorly ; anterior hiatus extending 

 from the base above, the middle of the valves, and from the front back 

 about one-fourth their length ; dorsal and ventral margins nearly straight 

 and parallel, the latter rounding up abruptly into the notch in front; beaks 

 strongly incurved, pointed, rather depressed, and placed near the ante- 



