84 Mr. W. King on certain Genera 



line, and the apex to the rostral point. Owing to the great in- 

 curvation of the beak in some species (P. Knightii), the aperture 

 is concealed, except in the young state ; but other species (P. con- 

 chidium), in which the beak is slightly incurved, have it exposed 

 during their entire existence. Nothing more need be said to 

 show that the aperture is the same as the open deltidium of Spi- 

 rifer, &c. From the sides of the deltidium two plates extend to 

 within a quarter of their length of the frontal margins of the 

 shell, at the same time decreasing in depth and gradually be- 

 coming more and more separated from the roof of the valve to 

 which they belong, till their extremity, which is reduced to a 

 mere point, is within an eighth of their length of the inner sur- 

 face or floor of the opposite valve (P. conchidium) . Both plates 

 are conjoined superiorly throughout their entire length ; and as 

 they follow the curve of the upper valve, though somewhat more 

 sharply, they form as it were a longitudinally curved arch-shaped 

 process, which strongly resembles the upper mandible of a parrot, 

 supposing the base of the mandible to be attached to the sides of 

 the deltidium. At their point of attachment to the cardinal 

 margin, the plates are thickened, or rather converted into two 

 condyles, which fit into a pair of sockets excavated in the corre- 

 sponding part of the opposite valve : in this mode of articula- 

 tion, Pentamerus agrees with all the dentigerous palliobranchiate 

 genera. 



Owing to the different degrees of incurvation of the beak in 

 different species, the arch at its posterior end, that is, where the 

 plates are attached to the sides of the deltidium, presents some 

 widely different appearances : thus in Pentamerus galeatus, in 

 which the beak curves so much downwards as actually to overlap 

 the natis of the opposite valve to some extent, the arch, from the 

 condyles to the rostral point or apex of the umbone, is doubled 

 up as it were ; whereas in Pentamerus conchidium, in which the 

 beak extends considerably behind the hinge line, the correspond- 

 ing part of the arch is completely unfolded. 



Besides being connected with the sides of the deltidium, the 

 arch is attached to the medio-longitudinal line of the roof 

 of the dorsal valve by means of a vertical plate extending 

 along its crest, from the posterior to nearly the anterior extre- 

 mity. The length and depth of this plate vary according to 

 species : in P. conchidium and P. Knightii, its superior margin 

 embraces the posterior three- fourths of the length of the shell; 

 but in P. galeatus and P. bashkiricus it extends no further than 

 the centre; and as the arch falls lower in P. Knightii and P. 

 galeatus than in P. conchidium and P. bashkiricus, this plate is 

 consequently deeper in the former than in the latter. 



The ventral valve (of Pentamerus galeatus) is furnished with 



