ANIMALS. 171 
Pl. 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 SEDGWICKIANA, 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, Pl. XIV, is supposed to be the 
small or under valve. 
Bakevellia Sedgwickiana is found, but rarely, in the Shell-limestone of Tunstall Hill. 
Family Arncip& (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 Arca Noe, Linneeus. 
As the first species, which the author of the Systema Nature described under his 
group Arca, 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 Arca Noe, and which, 
in the absence of the original diagnosis, I have made free in characterising as above. 
It is distinguished from <4rca in its regular form, approximating, by means of this 
character, to the regularly-formed, inequivalved, non-byssiferous Arcide—those repre- 
sented by the Arca antiquata of Linneus. Several genera of drcide, such as 
Litharea, Semla, Trisis, Barbatia, and others, have been proposed, some of which 
probably embrace the last-named species; but being unacquainted with their type, it 
s impossible for me to proceed further with a comparative view of the present genus. 
