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180 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



however, that the peculiar callus, or protuberance, mentioned by Mr. Conrai 

 as occurring under the anterior cardinal plate in four valves of the typic; 

 specimens examined by him, does not exist in a number of fine separate valves 

 collected at the same time and from the same locality as that at which his 

 types were obtained, and consequently is not a constant character. 



The shells included in the genus CaUista are often considered congen- 

 eric with Cytherea — more properly Meretrix, Lamarck — and although at one 

 time holding that opinion, I now believe that Dr. Gray, H. and A. Adams, 

 and others who have separated them under Poli's or other names, are right. 

 They are not only distinguished by the very faintly sinuous, or nearly simple 

 pallial line in Meretrix, as properly restricted, but by the siphons of the 

 animal in the latter group being only united half their length, and the mantle- 

 margins plain ; while in Cal/isfa, the margins of the mantle are plicated, and 

 above the siphons provided with filaments. The siphons also differ in being 

 united all the way to their extremities, which are crowned with cirrhi, instead 

 of being plain as in Meretrix. 



The question in regard to the particular name that should be retained 

 for the group under consideration is a rather complicated one, and conse- 

 quently quite different views are maintained on this point, even among those 

 who separate it from Cytherea or Meretrix, as here understood, some using 

 for it Dr. Gray's name Dlone, and others Gallista, Poli ; while those who 

 unite it with Lamarck's genus, in some instances use his name Meretrix, 

 and in others Cytherea. Until recently, I had not seen Poli's work (Test. 

 Utr. Sic), and as my friend Mr. Conrad had stated in one of his papers that 

 CaUista, Poli, was founded on the animal of Mactra, I was thus led to believe 

 that this name could not be properly retained for the group under considera- 

 tion, and adopted for these shells Dr. Gray's name Dione, ranging our Creta- 

 ceous species of the Upper Missouri under the same. Through the kindness 

 of Prof. Alexander Agassiz, I have since had an opportunity to consult Poli's 

 work, and find that it was in his first volume, published in 17tl, that he first 

 used the name CaUista ; and after a brief diagnosis of the animal, he there 

 only cites under it (1) Venus chione (2), V. deflorata, and (3) and last, V. 

 gallina, Linn. Consequently, it is to this volume and date that we must go to 

 ascertain to what group of shells his name CaUista belongs, as well as to 

 decide in regard to his type-species; and not to his second volume, published 



