142 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



ends v ; and also, that "the pallial sinus is deep, extending in the elongated 

 forms like Hecuba or Iphigenia, horizontally; in Donax and others it is 

 ascending and dilated.'' 



I think, therefore, from all that is yet known in regard to the characters 

 of Tancredia, that its provisional arrangement as the type of distinct family 

 is at least quite as apt to be correct as its introduction into a family in which 

 a decidedly and generally largely sinuous pallial line, and closed valves, are 

 conspicuous features. Indeed, I know of no reason for placing it in the 

 Donacidce, excepting the general external similarity of some of the species to 

 Donax ; and that this is not always a reliable means of deciding the question 

 in regard to the family-affinities of shells, is, to say nothing of other cases, 

 strikingly illustrated by Dr. Stoliczka's reference of Tellinomya, Hall, to the 

 Anatinidee*, and Cleidophorus to the Solemyidcs, when it is now well known 

 that both of these genera have crenated hinges, and belong to the Nuculidai; 

 although some of the species of the former agree more nearly in form and 

 general external appearances with some types of the Anatinidtz, than with the 

 usual forms of the Nuculidce. Nevertheless, as long back as 1857 (see Cana- 

 dian Naturalist of that year), Professor Hall fully illustrated the hinge of this 

 genus, and showed it to be clearly crenated in the type-species, T. nasuta 

 as well as in T dubia and T. cuneata ; while a number of the other species 

 referred to this genus are known to possess the same hinge-characters. The 

 same has also more recently been determined with regard to even the type 

 species of Cleidophorus. 



Tancredia Americana, M. & H. 



Plate. 38, figs. 1, a, h, c, d, e,f, g, h. 



Hettangia Americana, Meek and Hayden (Nov., 185G), Proceed. Acad. Nat. Sci. Philad., VIII, 274 ; and 

 (1860), ib., XII, 185. 



Shell thick, ovate-subtrigonal, moderately gibbous in the umbonal 

 region; posterior end broader than the other, obliquely truncated and gaping 

 above, rather narrowly rounded or subangular below ; anterior half narrow, 

 compressed, and presenting a more or less rostrated aspect, subangular, or 

 very narrowly rounded at the extremity; basal margin forming a broad semi- 

 ovate curve, being more prominent posteriorly and in the middle, than toward 

 the front; dorsal border elevated in the region of the beaks, from which it 



* Pateont. Iudica, III, G2 and 326. 



