Dr. J. E. Gray on some Families of Bivalve Shells. 417 



a. Mytilina. Hinder part of mantle only slightly produced ; 

 anterior muscle small. 



/8. Crenellina. Hinder part of mantle produced, forming false 

 siphons. Crenellida, Syn. B. M. 1842, 82. 



7- Lithodomina. Hinder part of mantle more or less produced ; 

 anterior muscle moderate-sized. 



7. Pinnada, Syn. B. M. 1842, 83 ; P. Z. S. 1847, 199. Anus 

 furnished with a long ligulate valve. 



Order Y. Pectixacea. 



Mantle leaves free all round, without any separate opening 

 for the outgoing current and vent. 



Suborder 1. Arcacea. Shell oblong or roundish, the anterior 

 and posterior adductor muscles subequal. Mantles bearded 

 behind. Hinge-teeth fleeply grooved or divided into tram- 

 verse interlocking plates. 



a. Foot lanceolate, subulate, angulated (for leaping). 



1. Tngoniada, Syn. B. M. 1842, 81; P. Z. S. 1847, 197. 

 Shell pearly within. Hinge- teeth two, diverging, deeply cross- 

 grooved. 



b. Foot truncated or dilatile at the end, often byssiferous (for 



anchoring) . 



2. Arcada, S\ti. B. M. 1842, 81 ; P. Z. S. 1847, 197. Foot 

 rather compressed, truncated, often secreting a laminal byssus. 

 Gills subpinnate. Shell subquadrate or roundish. Hinge-teeth 

 two, each divided into numerous transverse interlocking teeth. 

 Gills formed of fibres. 



a. Arcaina. Foot byssiferous. Gills subpinnate, separate from 

 each other behind. Hinge-teeth straight, even with the 

 hinge-margin. Cartilage in small marginal pits. 



y9. Pectunculina. Foot securiform, simple, not byssiferous ; gills 

 dependent. CartUage in small marginal pits. 



7. Nuculina. Foot compressed, end of disc bearded. Gills 

 pinnate; lips broad, triangular, large, striated internally. 

 Cartilage in a small, central, internal pit. 



Suborder 2. Malleacea. Shell subtrigonal ; anterior adductor 

 muscle small, rudimentary, hinder large, subcentral ; gills 

 laminar. 



3. Pteriada, P. Z. S. 1847. Aviculidae, Syn. B. M. 1842, 83. 

 Ann. i^ Mag. N. Hist. Ser.2. Vol. toil. 27 



