28 PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 



The internal cast also shows a deep groove, corresponding to a heavy 

 internal rib, extending back from the beak to the posterior margin above 

 the umbonal ridge, and some specimens show a much fainter furrow in the 

 same position on the exterior of the shell. 



On one specimen near the beak there is a small subtriangular structure 

 that appears to be an accessory valve (protoplax) and indicates by its form 

 that there were two of them, as in Xylophaga. 



The animal burrowed in wood, forming long more or less tortuous 

 shelly tubes like those of Teredo, the surface of the tubes bearing irregular 

 annular wrinkles, or lines of growth. 



Length of a medium sized specimen, 7 mm.; height, 6.5 mm.; convexity 

 of both valves, 6 mm. The larger tubes measure 7 mm. in diameter and 

 some of them, though broken, are over 30 mm. long. 



This species is quite similar in habit and general form to some species 

 of Teredo, such as T. torulosa Stoliczka from the Cretaceous of southern 

 India, but the apparent presence of a callum and of accessory valves and 

 the strong internal rib prevent its reference to Teredo. In the presence 

 of a callum closing the anterior hiatus, it differs also from the type of 

 Turnus, but in other characters, including the supposed " protoplax," it 

 agrees with that gemis, for although described as without accessory valves, 

 a specimen of the type species ( T. plenus] from the Cretaceous of Cotton- 

 wood Creek, California, shows a structure precisely like that described as 

 a probable protoplax in this species. The presence or absence of a cal- 

 lum in the adult is considered less important than the other features de- 

 scribed. 



Several fragments of fossil wood in the collection are filled with the tubes 

 and these have yielded 19 more or less perfect specimens of the shells. 



Locality and position. From mouth of cafion of Rio Tarde, four miles 

 east of Lake Pueyrrydon, Lower conglomerate, 300 feet below Ammonite 

 bed. 



SCAPHOPODA. 



DENTALIUM (L^EVIDENTALIUM) LIMATUM sp. nov. 



PI. VI, Fig. 9. 



Shell rather large, slightly arcuate, with circular cross-section; surface 

 appearing smooth and highly polished, but showing when magnified very 



