CYTHEREA. 397 



zones, have oftentimes (especially in young individuals, 

 and upon the umbonal region of the adult) an interrupted 

 appearance ; the beaks, too, have almost invariably three 

 short indistinct ray-like colourless markings proceeding 

 from them, of which the central is the shortest, and the 

 others margin the commencement of the lunule and the 

 ligament. The ventral outline is moderately and tolerably 

 evenly curved ; the declination of the hinder dorsal edge 

 is very moderate, and slightly convex ; that of the front 

 dorsal is stronger, incurved or retuse at first, and arched at 

 its termination. The anterior side, which occupies about 

 one-third of the shell, is attenuated at its extremity where it 

 is rounded both above and below ; the posterior side like- 

 wise tapers more or less, and is bluntly and unsymmetri- 

 cally rounded at its termination. The umbones project 

 very considerably forward, but are not swollen ; the beaks, 

 which are acute and inflected, are preceded by a very 

 large oval-lanceolate lunule, which is well defined, yet not 

 greatly depressed, and rises or pouts at the rather tortuous 

 seam. The ligament is long, moderately prominent below, 

 but overhung above by the projection of the valves at the 

 umbonal region, the hinder dorsal areas inclining inwards 

 but not forming any excavation or lozenge-like area. The 

 inside is of an uniform white, its margin is blunt and entire. 

 The hinge consists in the right valve of two central entire 

 primary teeth, (of which the front one is the thinner and 

 shorter, and the posterior shelves greatly behind) and an 

 extremely oblique somewhat bifid produced hinder one. 

 Before all these stands the receptacle for the front 

 sublateral lamina of the left valve, which latter has in 

 addition a central cloven but very erect and narrow pri- 

 mary tooth, behind which is a much more solid entire one, 

 and a third excessively oblique almost linear one adjacent 



