ANIMALS. 171 



PI. XIV, fig. 42) ; and, although stated to be a " nearly smooth" species, it is seen, 

 when examined by a common magnifier, to be prettily ornamented with very fine 

 raised lines. 



It occurs in the Shell-limestone of Tunstall Hill, where it is very rare. 



Bakevellia Sedgwtckiana, King. Plate XIV, figs. 38, 39, 40. 



Bakevellia Sedgwickiana, King. Catalogue, pp. 10, 11, 1848. 



Diagnosis. — Outline " somewhat acutely rhomboidal ; smooth ; pointed in front ; 

 and scarcely winged." (King.) 



The anterior and posterior margins of this species are remarkably oblique to its 

 cardinal line, which is caused by the slight development of the posterior part of the 

 hinge and the anterior lobe, together with the opposite or posterio-ventral portion of 

 the valves being very much produced. It is much flatter than any of the previously 

 noticed species. The specimen represented by fig. 39, PI. XIV, is supposed to be the 

 small or under valve. 



Bakevellia SedgwicJciana is found, but rarely, in the Shell-limestone of Tunstall Hill. 



Family Arcid^ [Arches), Cuvier, 1800. 



Diagnosis. — Valves variable in outline, according to genera. Hinge straight ; 

 furnished often its entire length with numerous interlocking teeth. Cartilage external ; 

 generally divided ; and situated on an area between the umbones. 



The family is here restricted to the true or areagerous genera. 



Genus Byssoarca, Swainson, 1820. 



Diagnosis. — Inequilateral : equivalved : generally wider than long. Ventral margins 

 sinuated for the passage of a byssus. 



Type Area Noce, Linnaeus. 



As the first species, which the author of the Systema Natura described under his 

 group Area, is the A. tortuosa, it follows that this singularly-formed shell must be 

 considered as the type of the Linnean genus : this circumstance renders imperative our 

 adopting Swainson's genus Byssoarca, which is typified with the Area Noa, and which, 

 in the absence of the original diagnosis, I have made free in characterising as above. 

 It is distinguished from Area in its regular form, approximating, by means of this 

 character, to the regularly-formed, inequivalved, non-byssiferous Arcida — those repre- 

 sented by the Area antiquata of Linnseus. Several genera of Arcida, such as 

 Litkarca, Senilia, Trisis, Barbatia, and others, have been proposed, some of which 

 probably embrace the last-named species ; but being unacquainted with their type, it 

 is impossible for me to proceed further with a comparative view of the present genus. 



