262 UNITED STATES GEOLOGICAL SUEVEY OF THE TERRITORIES. 



with each other; that is, where two are boring in directions that would 

 cause them to cross each other's paths, one or both instinctively turn aside, 

 leaving a more or less thin wall of wood between the two burrows. 



It has long been a matter of speculation, and quite different views are 

 yet entertained, in regard to the agency by which these little creatures manage 

 to penetrate the hardest kinds of wood, and some allied genera of mollusks 

 even hard rocks. It is now believed by many, however, that this is mainly, 

 if not entirely, accomplished by the attrition of the surface of the anterior 

 part of their valves, which is always covered by little sharp projections, 

 against the substance that they wish to penetrate. Others maintain that it is 

 by means of the muscular foot, that the animal excavates its burrows. 



As in many other cases, we cannot, in the present state of palseonto- 

 logical science, state very nearly at what period the Teredines made their first 

 appearance. Borings of wood, very similar to (hose of Teredo, have been 

 found even in the Carboniferous rocks, and referred to this genus (T antiqua, 

 McCoy) ; also, from the Jurassic ; but there is no certainty that the mollusks 

 that formed these borings would fall properly into the genus Teredo as 

 restricted by modern conchologists. Iudeed, the probabilities are that the 

 Palaeozoic species, at least, would be found to present generic, if not greater 

 differences, if we had the means of knowing all of their characters. In the 

 Cretaceous, however, we find not only the burrows, but the delicate valves ol 

 a number of species that agree so nearly with the corresponding parts of the 

 modern typical species of Teredo, as to leave little or no room for doubts in 

 regard to their belonging to that, or some very nearly allied genus. It is 

 also represented in the Tertiary rocks, and in our existing seas and estu- 

 aries. The known living species are not very numerous ; but it is probable 

 that the genus nearly or quite attains its greatest development at the pres- 

 ent time. Although the number of known recent species is not very 

 great, vast numbers of individuals exist in almost all seas; the geuus having 

 a nearly world-wide distribution. 



Teredo selliforinis, M. & H. 



Plate 17, figs. 19, a, b, c, d. 

 Teredo selliformis, Meek and Ilajden (May, 1860), Proceed. Acad. Nat. Sci. Philad., XII, 178. 



Shell subglobose ; posterior side narrowly rounded above, and provided 

 with a broad angular notch below ; anterior ventral side having a very large 



