Report of the State Geologist, ;">7 1 



subcentral or subposterior. Pedicle-valve with a small septum 

 ;i> in DisoiNA, behind which is an impressed disc or area, exter 

 nally concave and internally elevated. This is perforated by a 

 Longitudinal fissure, extending from a short distance behind the 

 septum nearly to the posterior margin, which is often slightly 

 indented behind it. . Shell more or less horny in texture, 

 minutely tubulose. Brown or yellowish in color. Animal with 

 labial folds united. Spiral arms free, their apices being directed 

 toward the pedicle-valve. Oral cirri long and flexible ; setaa of 

 the mantle stiff and brittle. Adductor muscles, four; sliding- 

 muscles also four. 



Type, Discmisca lamellosa, Broderip (sp.). 



Distribution. American seas (Atlantic, Pacific. Carribean), 

 China. Fossil: Mesozoic (?) to Tertiary. 



Discina, Lamarck. 1819. 



Synonym. Orbiotda^ Sowerby. 1818 (not Cuvier. 1798). 



Shell orbicular, calcareo-corneous, thick, impunctate, lamellar. 

 Valves convex. A small, sharp, longitudinal septum rises from 

 the center of the lower or pedicle- 

 valve, of a subtriangular shape, covering ^^^^a aa 

 and hiding a small tubular perforation 



Of the apex Of the Shell. This perfor- Fig. 264. -Vertical section of the pedi- 



. cle-valve of Discina striata. After 



ation is very oblique, and from its in- Davidson. 

 ternal opening a groove extends backward nearly half-way to the 

 posterior border of the shell. The anterior muscular scars 

 meet in front of the septum and form a semilunar elevation with 

 the points directed backward. The posterior scars in the lower 

 valve are small and widely separated. On the external surface 

 the foramen appears nearly in the middle of the shell and the 

 furrow is continued anteriorly for a short distance. Upper or 

 brachial valve convex, apex subcentral ; a slight median longi- 

 tudinal callus internally. There is no strongly impressed 

 disk about the foramen as in Discinisca; color brownish 

 yellow. 



Type, Discina striata, Schumacher. 



Habitat. Cape Palmas, West Africa. (Recent: but one 

 species known.) 



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