MONOMYARIA. 123 



This genus contains two sections : — * Shell without furrows or striae. 

 ** Longitudinally fuiTowed. 



The distinction between this genus and that of Anomia, is that in 

 the latter the perforation is always in the smaller valve, while in the 

 Terebratula it is in the larger one. There is, however, a still greater 

 distinction in the animals. 



Terebratula Cranium. — The Skull Terebratula. 

 Plate XIV. fig. I. Slightly ovate, ventricose ; summit of 

 the convex valve produced and blunt; wdth divergent striae, 

 and the margin crenulated. Half an inch long. Inhabits 

 the sea at the Zetland Islands. 



Genus 3.— ORBICULA — Lamarck. 



Generic Character. — Suborbicular, inequivalve ; without 

 apparent hinge ; under valve very thin, depressed, and ad- 

 hering to marine bodies ; upper valve subconic, \vith the 

 vertex acute, and more or less elevated. 



Orhicida Norwegica The Norwegian Orbicula. 



Plate XIV. fig. 32r^ Upper valve in the form of a depressed 

 cone, with a produced and pointed summit. Inhabits the 

 Norwegian seas. v ^ 



TRIBE II. — RUDISTA. 



Animal unknown, as are also the ligament and hinge; 

 shell with very unequal valves, and having no distinct 

 umbones. 



Genus 4. — CRANIA. — Bruguiere. 



Generic Character. — Inequivalve, suborbicular ; under 

 valve flattened, fixed below, and perforated on its inner sur- 

 face by three unequal oblique holes ; upper valve very con- 

 vex ; provided interiorly with two projecting callosities. 



Crania personata. — The Masked Crania. Plate XIV. 

 fig. 5. Orbicular ; the upper valve more gibbous and some- 

 what conical; lower valve flat, with three pits. Inhabits 

 the Indian ocean. 



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