Thetis. 



CONCHIFERA. 



225 



Genus LV.— CRASSATELLA.— Zamarc/t. 



Shell thick, equivalve, transverse, inequilateral ; external 

 surface generally covered with a brown horny epidermis, and 

 more or less transversely grooved ; one valve provided with 

 two strong cuneifurni, rugose, cardinal teeth, which are some- 

 times perpendicularly grooved, and one primary tooth in the 

 opposite valve ; lateral teeth awanting or nearly obsolete, two 

 strong oblong depressions, the one on the anterior side of the 

 umbo somewhat elongated, and not so well marked as that in 

 the posterior side ; two remote, lateral, rather oblong muscu- 

 lar impressions ; ligament internal, attached to a concave pit 

 situate on the anterior side of the hinge, this space is divided 

 by a rib into two portions, the outer half of the ligament is 

 externally visible when the valves are closed. 



1. Crassatella sulcata. — The Furrowed Crassatella, pi. 

 XC. %. 31. 



C. sulcata. Sowerby, IV. p. 62, pi. 345, fig. 1. Tellina 

 sulcata, lirander, fig. 89. 



Ovate, transversely elongated ; anterior side produced, ob- 

 liquely truncated, and defined by a moderately distinct ridge ; 

 posterior side rounded ; surface covered with transverse ribs 

 with deep intervening furrows, which are but faintly marked 

 on the truncated side ; beaks rather pointed ; internal edge 

 crenated. 



The London Clay, Barton. 



2. Crassatella plicata. — The Plaited Crassatella, pi. 

 XC. fig. 22. 



C. plicata. Sowerby, IV. p. G2, pi. 3-)5, fig. 2. 



Oblong-ovate; anterior side defined b\' an oblique, obtuse 

 ridge, and slightly truncated ; whole surface with numerous 

 fine transverse plaits ; margin crenated within. 



The London Ciay, Barton. 



3. Crassatella compeessa. — Tlie Compressed Crassa- 

 tella, pi. XC. fig. 36. 



C. compressa. Lamarck, An. du Mus. 6, p. 410, pi. 20, 

 fig. 5. Deshai/es, Coq. Fos. pi. 3, figs. 8, 9. 



Sub-triangular ; anterior si<le shertest and rounded ; pos- 

 terior side nearly straight from the two-thirds downwards, 

 from thence obliquely truncated ; the diagonal ridge termin- 

 ating in an acute angle ; surface with many transverse narrow 

 furrows, which become abruptly angular after passing the ridge ; 

 internal margin destitute of crenulations. 



Kellia sul-orlicularis. Turton, Brit. Bio. p. 57, pi. 11, 

 figs. 5, 6. 



Sub-orbicular, very convex, thin ; beaks nearly central, and 

 slightly inflected ; sides nearly equal and rounded ; basal mar- 

 gin rather straight, a single tooth in one valve immediately 

 under the beak, locking into a double incurved one in the 

 other ; with a laminated tooth behind the umbo in each valve. 



The Pleistocene Marine Formation, Largs, and the Coral 

 Crag, Sutton. 



<}enus LVIL— MONTACUTA.— 7'Mrto«. 



Shell oval or oblong, equivalve, inequilateral, mostly closed ; 

 hinge with two teeth in each valve, and a cavity between them ; 

 destitute of lateral teeth ; ligament internal. 



1. MoNTACUTA suB-STRiATA. — The Sub-striatcd MontacutH. 

 M. suh-striata. Turton, Brit. Biv. p. 59, pi. 11, figs. 10, 11. 



TdVxmya. Brown, 111. Rec. Conch. Brit. p. 107, pi. 40, f. 23. 



Sub-ovate, somewhat oblique, a little iuflated, slightly con- 

 tracted in the middle of the valves ; beaks prominent, straight, 

 and not quite central ; anterior side large and rounded, the 

 other short and narrowed ; surface with nearly obsolete dis- 

 tant striae. 



The Coral Crag, Sutton. 



2. MoNTACUTA GLABRA. — The Smooth Montacuta, pi. XC. 

 fig. 32. 



Tellimya glabra. Brown, 111. Rec. Conch, p. 107, pi. 42, 

 figs. 20, 21. 



Elliptical, moderately convex, thin, and smooth ; anterior 

 side rounded, and posteriorly sub-truncated ; beaks placed 

 considerably to one side ; one broad primary tooth in eac^i 

 valve ; with a central hiatus. 



The Coral Crag, Sutton. 



3. MoNTACUTA PERRUGINOSA. — The Rusty Montacuta. 



M. ferriiffinosa. Turton, Brit. Biv. p. 60. Tellimya ellip- 

 tica. Brown, III. Rec. Conch. Brit. p. 106, pi. 42, figs. IG, 17. 



Transversely sub-ovate, moderately convex, with obsolete 

 concentric wrinkles ; beaks obtuse, sub-central, basal margin 

 nearly straight ; hinge with two projecting teeth, one of which 

 is erect, the other slopes inwards and downward, separated by 

 a triangular hiatus. 



The Pleistocene Marine Formation, Ireland. 



Genus LVL— TELLl M Y A .—Brotcn. 



Shell equivalve, slightly inequilateral ; sub-orbicular; con- 

 vex ; close all round ; left valve destitute of cardinal teeth, but 

 provided with a pretty large hiatus, and two projecting later- 

 al teeth, each having a groove in its centre ; sometimes with 

 one or two rather long teeth on the right side ; right valve with 

 two recurved, prominent teeth, which occupy the vacant space 

 below the beak in the opposite valve ; muscular impressions 

 large and distinct; pallial impressions entire; ligament internal. 



1. Tellimya suB-onuicuLAUis. — The Sub-orbicular Telli- 

 mya, pi. XC. figs. 34, 35. 



Genus hVlU.— THETIS.— Soicerby. 



Shell bivalve, equivalve, sub-equilateral, more or less orbi- 

 cular and convex ; ligament marginal ; hinge with three or 

 four acuminated teeth ; line of attachment of the mantle (?) 

 with a deep sinus, extending nearly to the beak ; muscular 

 impressions round, small, and remote from the hinge. 



1. Thetis minor.— The Small Thetis, pi. XCII. f. 3, 4, 5. 



T. minor. Sowerby, VI. p. 21, pi. 513, figs, 5 and C. 

 Corlula laevigata. lb. p. 14, pi. 209, figs. 1 and 2. Venus, 

 Mantell. Geology of Sussex, p. 73, No. 12. 



Shell gibbose, wider than long ; beaks pointed, nearly ap- 

 2 z 



