BIVALVIA. 55 



2. MYTILUS HESPERIANUS, Lamarck. Tab. VIII, fig. 10. 



MYTILUS HESPERIANUS. Lam. Hist, des An. Sans. Vert., t. vi, p. 127, 1819. 

 Desk. 2d ed. Lam., t. vii, p. 48, 1836. 

 Payr. Cat. Moll. Cors., p. 68, pi. 2, fig. 5, 1826. 

 DENSATUS. S. Wood. Catalogue, 1840. 



Spec. Char. Testa, elongatd, obliqud, incurvatd densatd, crassd ; margine dor soli 

 arcuatd. 



Shell elongate, oblique, incurved, thick and heavy ; dorsal margin convex. 



Greatest Diameter, 2^ inches. 



Locality. Cor. Crag, Sudbourn. Recent, Mediterranean, and Coast of Spain. 



Two specimens only of this shell have yet come into my possession, and these are 

 both of the right valve, and as they are all that I have seen, it does not appear to have 

 been abundant during the Cor. Crag period. Fragments of a very thick Mussel, 

 indicating a considerable curvature, and which I presume to belong to this species, 

 are not unfrequently met with in various parts of the Red Crag ; and as they have 

 undergone a considerable deal of bouldering, may possibly have been washed out of 

 the deposit of the antecedent period. I have given it therefore as a certain inhabitant 

 of the older formation only. 



It appears to present characters different from any of the varieties of the common 

 edible Mussel, sufficiently it is presumed to entitle it to be considered a distinct species. 

 The variety called Myt. incurvatus, Mont., approaches nearest in form, but the dorsal 

 margin is never so convex as in our shell, and the specimens I have seen are much 

 thinner. The Crag shell is very thick, more especially in the narrow part near the 

 beaks, and the anterior side curves inwardly, while the dorsal and posterior portions are 

 particularly convex in outline ; the umbones are eroded, and the outer part of the shell 

 near the beaks is so thin as to show the white lining through it, while towards the 

 ventral portion the shell is of a deeper colour, as described by Payraudeau. My 

 specimens are destitute of hinge-teeth, but their probable habitat in deep and more 

 tranquil water may have rendered such unnecessary, and they may have thus become 

 obsolete. 



MODIOLA,* Lamarck, 1801. 



VOLSELLA. Scopoli, 1777, sec. Gray. 



CALLITRICHE et CALLITRICHODERMA. Poll., 1/95. 



AMYGDALCM. Megerle, 1811. 



CRENELLA. Brown, 1827. 



BRACHYDONTES. Swains., 1840. 



LANISTES. Id. 1840. 



MODIOLARCA. Gray, 1840. 



MODIOLARIA. Beck, sec. Loven, 1846. 



LANISTINA. Gray, 1847. 



MODIOLOPSIS? Hall, 1847. 



* Etym. Modiolus. 



