Dr. J. E. Gray on some Families of Bivalve Shells. 41 



7. Cardissa, Megerle, Gray, P. Z. S. 1847, n. 558. Shell cor- 

 date, keeled, very short, hinder side shortest or impressed; 

 lunule simple; cardinal teeth distinct, more or less distorted. 

 C. Cardissa. 



8. Lunulicardia. Shell cordate, keeled; lunule deeply im- 

 pressed ; cardinal teeth more or less distorted. L. retma. 



Cardium avicularia and C. cymbulare are more allied to Hip- 

 popus than to Cardium; like Hippopus the front side is short, 

 with the gaping edges, and not the hinder as in Cardium, and the 

 teeth are obHque and not placed in a cross. If distinct from 

 Hippopus, it may form a genus of Tridacnidce, named Avicu- 

 larium, characterized by the height of the shell (see Gray, P. Z. S. 

 1847, 561). 



Adacna {laviuscula) has a long and Didacna a small siphonal 

 inflection and distinct siphons ; they are evidently more allied 

 to the genera Panopea and Cyrtodaria than to Cardiada, with 

 which they have generally been placed, apparently on account of 

 their costated shells. 



Fam. 8. Mactrad^, Gray, Syn. B. M. 1840, 137; 1843, 75. 

 Shell equivalve. Cardinal teeth two in each valve, the hinder 

 one small, compressed, often rudimentary, the front one trian- 

 gular, more or less deeply nicked; lateral teeth of left valve 

 simple, of right double. Cartilage in an internal pit. Siphonal 

 niflection distinct. Mantle lobes more or less free beneath, 

 united before and behind, and extended into two retractile 

 siphons. Foot lanceolate, subanterior. 



A. Shell subtriangular, ovate, nearly closed behind; lateral teeth 

 distinct, well developed, laminar ; mantle lobes free. Mactrina. 



a. Ligament in a groove above the cartilage-pit. 



l.Schizodesma, Gray, Mag. N. H. i. 370; P. Z. S. 1847, 

 n. 563. Shell triangular ; lateral teeth simple, compressed, s'. 



Spevgleri. 



b. Ligament marginal, triangular, separated from the cartilage- 



pit by a shelly ridge. 



2. Mactra. Mactra A., Gray, Mag. N. H. i. 370.' Shell tri- 

 gonal ; lateral teeth elongate, Unear, subequal. M. stultorum. 



3. Mactrinula. Mactra C., Gray, Mag. N. H. i. 371-A Shell 

 trigonal, thin ; hinge-margin double ; lateral teeth short, very 

 close to the cardinal ones. M. plicaria. 



4. Mactrella. Mactra B. & E., Gray, Mag. N. H. i. 37l. 

 Shell cordate, triangular, thin ; hinder lateral teeth very short, 

 rudimentary, and near the cardinal. M. striatula. 



i^ 



