OF MOLLUSCA. 185 



**** Margin bald, smooth, covered with a hard shining skin be- 

 neath, entire behind. 



14. Tonicia. 



Margin naked, smooth ; valves broad, transverse, keeled, close 

 together ; margin of insertion pectinately toothed, of central valves 

 with a notch on each side. Gills occupying the greater part of the 

 sides. 



1. T. atrata, Sow. III. f. 57, 58. Falkland Islands. 



2. T. elegans, A. Adams, Gen. Moll. t. 53. f. 5 ; Sow. III. f. 73, 74. 



Conception Bay. 



3. T. rubra. 



4. T. fulva, Sow. El. f. 53. Cadiz. 



5. T. lineata, Sow. III. f. 77. 



6. T. Swainsonii, Sow. III. f. 5. Peru. 



7. T. cerasina. 



8. T. laevigata. 



9. T. lvrata. 



10. T. Grayii, Sow. III. f. 8, 16. 



1 1. T. castanea, Sow. III. f. 114, 115, 116. Cape of Good Hope. 



12. T. fastigiata, Sow. III. f. 11. California. 



15. Fannyia. 



Mantle broad, smooth ; valves moderate, subcordate, rounded and 

 far apart on the sides ; lobes of insertion wide, denticulated, of the 

 central valve with a single lobe on each side. 



1. F. dispuncta, Zool. Journ. t. 77. f. 5. 



This genus forms a passage to the genera which have only a small 

 part of the valves exposed. 



b. The exposed part of the valves small ; the plate of insertion very 



broad, of the anterior valve divided by deep grooves into several 

 lobes. 



16. Katharina. 



Body convex. Mantle smooth, naked ; exposed part of the valves 

 small, cordate ; plates of insertion very broad, large ; of the anterior 

 valve divided into several deep grooves above, and slit on the edge; 

 of the last three-lobed. Gills on each side of the foot. 



1. K. tunicata, Middend. Ross. Chit. t. 10. f. 1, 2. 



2. K. Douglasiae. 



c. The valves entirely covered by the mantle, appearing only to con- 



sist of theplates of insertion; the front part of the front valves 

 five-lob ed. 



17. Cryptochiton. 



Mantles covered with numerous close, uniform tufts of calcareous 



