INVERTEBRATE PALEONTOLOGY. 141 



In regard to the family-affinities of this genus, rather widely different 

 opinions have been expressed. In 1864. on comparing it and ihe allied genus 

 Meekia with all of the different families that might be supposed to be more 

 or less nearly related, and finding that, as these groups have been defined by 

 the highest authorities on conchology, these shells could not be admitted 

 into any of them, I proposed to separate them provisionally in a distinct 

 family to themselves, until, as stated, "the affinities of some allied extinct 

 types can be better determined.'' For this, 1 have been rather freely criti- 

 cised by Dr. Stoliczka, of the Indian survey,* who places Tancredia in the 

 Jtoiiacidte, and yet doubtfully refers, for some incomprehensible reason, the 

 very closely-allied genus Meekia to the Trigoniidce. 



In this connection, it is only necessary for me to remark that at least 

 quite as good authorities have placed Tancredia in several other more or less 

 widely removed families. For instance, Terquem thought it most nearly 

 allied to the Cardiidce, to which Pictet. and some others refer it. Woodward, 

 on the contrary, includes it. doubtfully in the Ludnidce, under which the 

 well-known and able conchologist, Dr. P. P. Carpenter, mentions it in his 

 Smithsonian lecture. Again, Chenu places it in the Isocardiidce, while, as 

 already stated, Dr. Stoliczka insists that it belongs to the Donacidce. From 

 this very diversity of opinion, however, it must be evident that this genus 

 presents a peculiar combination of characters that renders the determination 

 of its family-affinities quite difficult. 



The objections to placing it with the Donacidce are not only its simple 

 pallial line, but also the fact that in some of the species the valves are clearly 

 gaping behind ; that is, not merely a little parted, as we often see in the 

 Tellinida}, but show a decided gape, with the margins closed together above 

 and below -the hiatus. I am well aware that these are not necessarily always 

 family-characters; but every naturalist knows that a peculiarity which in 

 one groujj is scarcely of generic importance, may in another form a constant 

 family-character. All of the existing genera that are known by the anatomy 

 of the animal to belong to the Donacidce, have a well-defined, and generally 

 large pallial sinus, and closed valves ; and the group is so characterized by 

 the best authorities on conchology. Indeed, Dr. Stoliczka's own description 

 of the family Donacidce would exclude Tancredia, as he distinctly states, in 

 describing this family, that the shell is " always strong and solid, closed at both 



* Paheont. Iudica, III, i:W. 



