192 MAGAS. 



tracea, Lam. Conchacea, Bl. — Descr. Usually thin, equivalve, 

 sub-equilateral, sub-trigonal, slightly gaping at the extremities ; 

 hinge with one cardinal tooth, divided into two parts, diverging 

 from the umbo, with sometimes a very small laminar tooth close 

 to its side ; a deep triangular pit near the centre, containing the 

 cartilage ; one long, lateral tooth on each side of the umbo in 

 one valve, received between two in the other ; muscular impres- 

 sions two, lateral ; palleal impression with a small sinus. — 06s. 

 This genus contains many species of beautiful shells found in 

 various parts of the world, some of which are common in Britain. 

 Possil species are not numerous, they occur in the tertiary strata. 

 The principal forms are represented by M. Splengleri, fig. 81. 

 (Schizodesma) M. fragilis, fig, 80, (Spistjla) M. bicolor, fig. 

 82, (Mulikea) and M. Stultorum, fig. 79. PI. iv. fig. 79 to 82. 

 MACTEACEA. Lam. A family of the order Conchifera Dimy- 

 aria, Lam. Sect. Tenuipes. The cartilage placed in a trigonal 

 pit with a small external ligament. The genera may be thus 

 distinguished. 



1. Lutearia. No lateral teeth, shell gaping. The short 



species constitute the genus Ligula. Fig. 77, 78. 



2. Mactea. Lateral teeth, shell closed. This genus has 



been divided into Mactra, Mulinia, Schizodesma and Spi- 

 sula, by Mr. Gray. Pig. 79 to 82. 



3. Gnathodon. Teeth serrated, thick, one angular. Pig. 83. 



4. Ceassatella. Shell thick, lateral teeth. Pig. 84. 



5. Amphidesma. a distinct external ligament, internal liga- 

 t ment oblique. Pig. 85. 



6. Eetcina. a short tooth on each side of the cartilaginous 



pit in each valve. Including Mesodesma. Pig. 86. 



7. IJNGULiiirA. Ligament flat, divided. Pig. 88. 

 MACULATED. (Prom Macula, a spot.) Spotted or patched. 



This term is applied by conchological writers, to those shells 

 which are coloured in spots or small patches. In the same sense 

 it is also used aa a specific name. As for instance, Cytherea 

 maculata, fig. 167, c. and Hippopus maculatus, fig. 156. 

 MAGAS. Sow. (Mayae, magas, a board, a deck.) Fam. Brachio- 



