Dr. J. E. Gray on some Families of Bivalve Shells. 25 



Lucinidce. — As our knowledge of the animals of Univalve 

 shells has increased, we have found that shells which have a 

 great resemblance to each other are formed by very different 

 animals, until it has become almost impossible to pronounce 

 with certainty on the genera of several Gasteropodous Mollusks, 

 unless we are in possession of the animal and operculum as 

 well as the shell. The same fact is every clay forcing itself on 

 our notice with respect to the Bivalves. It is nearly impos- 

 sible to separate the Muteladce from the Uniones, the Modiolurcce 

 from the Modioli, though the animals are very unlike. 



All conchologists considered that the Lucinidce were a very 

 natural group, yet we learn that Ungulince of Daudin, which are 

 scarcely to be separated from them, except by their irregular out- 

 line from living in holes in rocks, have four gills and distinct labial 

 palpi, while the Lucince have only two gills and no labial palpi. 

 This appeared so improbable when 1 printed my paper in the 

 May Number of the ' Annals/ that I placed a mark of doubt 

 after the description, but I have since had an opportunity of 

 verifying the accuracy of the observations. 



One of the most striking instances occurs in the shell referred 

 to the genus Mysia, or Diplodonta. In the paper above referred 

 to I described the animal of a Philippine species of this genus, 

 which has two siphonal apertures and a lanceolate foot, and 

 referred it to the suborder Veneracea. M. Mittre in ' Journ. de 

 Conchyliologie/ 1850, t. 238, described and figured the animal 

 of a Brazilian species, which he calls Diplodonta Brasiliensis, 

 having only a single anal siphonal aperture and a cylindrical 

 foot like the Ungulincn; and which, indeed, appears chiefly to 

 differ from that genus in the anal aperture being further from 

 the pedal one, and in the adductor muscle being roundish in- 

 stead of linear and elongate : the difference in form of this part 

 probably explains the relative position of these two apertures. 



The examination of the animal of Ungulina, and M. Mittre' s 

 description and figure, show the necessity of forming for these 

 genera, as recommended by M. Mittre, a family, which may be 

 called Ungulinadce, characterized by the single anal siphonal aper- 

 ture, and the presence of two pairs of gills and distinct labial 

 tentacles, which will contain the genera, 1. Ungulina, Lamk., 

 2. Scacchia of Philippi, and 3. a new genus which may be called 

 Mittrea, having Diplodonta Brasiliensis for its type. One 

 of our English shells, Tellina rotundata, Montague, has been 

 referred to the genus Diplodonta, but I have not been able to ex- 

 amine its animal, and according to the description of Mr. Clark, 

 quoted by Messrs. Forbes and Hanley, it differs essentially from 

 any of the preceding : " the mantle plain, somewhat closed pos- 

 teriorly and anteriorly, but with a large opening for the foot in 



