76 SYNOPSIS OF THE CHARACTERS OF THE 



fossil bears to the Lithodomus dactylus, Cuv., which perforates the coral reefs of the Indian ocean, that were it 

 not that the fossil is a little more angulated posteriorly, it would be difficult to distinguish them even as 

 varieties ; the length is rather greater than the depth of the two valves in the fossil species ; the shell is very 

 tliin, and the concentric lines of growth obtuse and irregular; the oblique strise from the beak to the ventral 

 margin sharp, fine. From the cardinal angle to the opposite margin eight lines ; width one inch eleven lines. 



Lanistes. Humph. 



Gen. Ch. — Shell oval, transversely cordiform, ventricose; umbones prominent, with diverging elevated 

 striae ; hinge-margin not elevated or angular ; teeth none. 



The few mountain Hmestone shells I have referred to this genus, diifer from the recent species in having 

 the hinge-margin a little more elevated as in Modiola ; they have, however, the rugged, thin, inflated shell and 

 peculiar aspect of this genus. 



Lanistes obtusus. M'Coy. (PI. XIII. fig. 9). 



Sp. Ch. — Transversely ovate, obtusely roimded ; anterior end narrow, separated from the body of the 

 shell by a deep sinus in the ventral margin, from whence an oblique, shallow groove extends to the beak, 

 behind which the body of the shell becomes suddenly more gibbous; hinge-line short; posterior end broad, 

 rounded ; surface radiated with very numerous, fine, sharp, rough, slightly flexuous stria, frequently interrupted 

 by irregular wrinkles of growth; the striae are alternately larger and smaller towards the margin of large indi- 

 viduals, but are equal in size in small specimens. 



Distinguished from the Z,. rugostis, M'Coy, by its more obtuse form, distinctly lobed, anterior side, shorter 

 liinge-line, and very much sharper, finer, and more numerous radiating ridges. Length of most perfect speci- 

 men two Unes and a half, width four fines. Imperfect specimens are not uncommon, ten fines in length. 



Lanistes eugosus. M'Coy. (PL X. fig. 8). 



Sp. Ch. — Transversely ovate, shell thin, rugged; anterior end very short, rounded; umbones prominent; 

 posterior side dilated, angulated ; surface with numerous, coarse, flexuous ridges, from the beak to the posterior 

 margin. Greatest length from the cardinal angle to the opposite margin three lines, width six fines. 



Mytilus. Linn. 

 Gen. Ch. — Equivalve, cuneiform, oblique ; smooth; beaks terminal, pointed. 



Mytilits COMPTUS. M'Coy. (PI. XIII. fig. 12). 



Sp. Ch. — Acutely ovate, depressed; beaks terminal, acute, slightly incurved; dorsal margin convex, pos- 

 terior angle very obtuse ; front margin obfique, rounded ; anterior face truncate, concave towards the beak ; 

 surface smooth, with a few fines of growth. 



This fittle species difiers from the M. priscus of the Silurian rocks, in its more acute and curved beaks, the 

 greater strength of the ridge bounding the anterior face, its more convex and shorter dorsal margin, and more 

 obfique front margin. Length eleven lines, greatest width six and a half lines. 



Mytilus Flemingi. M'Coy. (PI. XI. fig. 29)- 



*S^. Ch. — Trigonal, length twice the width; hinge-line half the length of the shell, depressed, smooth; 

 anterior face truncated, bounded by a sfightly sigmoidal ridge ; beaks acute, terminal, curved towards the an- 

 terior side ; posterior margin convex. 



This species, fike the Dressina polymorpha, has the truncated anterior face and ridge flexuous or sigmoidal. 

 Length one inch four fines ; greatest width seven fines ; depth of anterior face three lines. 



