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



one on each side the upper part against the side of the body. 

 Mantle with a small valvular hole under the siphons. Hinge-teeth 

 rudimentary. Cartilage internal, in a pit in each valve, with a 

 peculiar shelly plate before it or covering it. Shell inequivalve, 

 umbo with a cartilage slit. Periostraca often hispid. 



a. Siphons united. Valves equal. Clavicle linear. Laternula. 



b. Siphons separate at the end. Valves unequal. Clavicle flat, 



covering the cartilage. Lyonsia, Byssonia. 



c. Siphons separate. Valves unequal. Clavicle small, suhcy- 



lindrical. Thracia, Periploma, Cochleodesma, Myodora, 

 ? Poromya, Near a. 



di. Siphons separate, one valve attached. Clavicle large. Chamos- 

 trea, Myochama. 



5. Hinge toothed or toothless. Cartilage external, marginal. Shell 

 pearly. Periostraca hard, polished. 



15. Muteladce, P. Z. S. 1847, 197. Iridinidse, Syn. B. M. 

 1842, 80. Foot large, compressed, angular in front; pedal 

 opening very long. Gills large. Shell scarcely to be distin- 

 guished from Unio or Anodon, but the hinder submarginal im- 

 pression is rather more truncated. 



II. Foot conical, acute, angularly bent behind (for leaping). 



16. Cardiadce, Syu. B. M. 1842, 75; P. Z. S. 1847, 185; Ann. 

 and Mag. N. H. 1853, 37. Gills narrow, elongate, floating, 

 only attached at the upper end. Cardinal teeth conical, forming 

 a cross when the valves are closed. Cartilage external, marginal. 



III. Foot truncated and dilatile at the end (for crawling and 



anchoring). 



17. Ledada. Nuculidfe §, P. Z. S. 1847, 190. Gills small, 

 oblong, subpinnate, formed of separate threads ; lips large. Palpi 

 very long, subulate. Siphons small, united. Hinge-teeth two, 

 nearly parallel with hinge-margin, each divided into numerous 

 transverse hook-hke plates. Siphonal inflection distinct. 



a. Cartilage internal, triangular. Leda, Yoldia. (See fig. 2.) 

 y9. Cartilage external, marginal. Solenella. 



17*. Modiolarcada. Gills four, thick, subtrigonal, truncated 

 in front, narrow, produced and united together behind the foot. 

 Lips four, moderate. Palpi obsolete. Siphonal apertures distinct, 

 anal moderate ; inhalant very large, inferior, simple-edged. Foot 

 oblong, truncated, lanceolate, acute in front, with subposterior 

 central hole for byssus. Shell equivalve. Hinge-teeth none 



