BIVALVIA. 167 



the exterior is more finely ridged over the centre, but the anal ridge beyond the siphonal 

 slope is naked. 



Fig. 12, PI. XXIV, represents a shell from Bracklesham, called by Mr. Edwards 

 planiuscida. This is more elongated, more inequilateral, and more compressed, and 

 covered with finer and sharper ridges. When more specimens are found it may probably 

 prove to be distinct. For the present I have inserted it here as a variety. 



7. Crassatella plicata, /. 8owerby. PI. XXIII, fig. 14 «, b. 



Crassatella plicata, J. Sow. Min. Conch., pi. 345, fig. 2, 1822. 



— — Nyst. Coq. Foss. de Belg., p. 85, pi. xli, fig. 3, 1843. 



Spec. Char. Cr. Testa transversa, elongato-trigond, inaquilaterali, compressiusculd ; 

 pedi-regione altiore ; alter d elongatd et truncatd ; extus sulcatd, sulcis posticis evanescentl- 

 bus ; umbonibus mmimis acutis, depressiusculis ; lunuld ovatd,profundd; siphoni-regione 

 depresso, laiiceolato ; carditis angusto ; dentibws parvis, simplicibiis ; marginibus in medio 

 tenuiter crenulatis. 



Shell transverse, elongately trigonal, inequilateral, somewhat compressed ; pedal 

 region the higher; siphonal side produced and truncated, covered externally with con- 

 centric ridges, which become obsolete over the anal region, where it is nearly smooth ; 

 hinge rather narrow, and inner margin crenulated in the middle. 



Length, \ an inch ; height, f ths of an inch. 



Localities. Bracklesham and Barton [Edwards); Southampton (/. Sow.). 



This was originally separated from sulcata by Mr. J. Sowerby, and I have retained it 

 as a distinct species. M. Deshayes has given this name as a synonym to Grignonensis ; 

 but the shells appear to be specifically distinct. Our present species is more finely ridged ; 

 it is shorter and rather more rounded in outline, with the siphonal region less angular. 



A small specimen, which, I think, must belong to this species, has very recently been 

 found by Mr. Alfred Bell in the modern deposit at Selsey, called the " Mud-bed." This is, 

 in all probability, a specimen washed out of the contiguous Eocene Formation. It much 

 resembles our present species, more so than any other known to me, but the ridges upon 

 the umbonal region are fewer and more distant than on the body of the shell, contrary to 

 the general character of the exterior ornament in this genus. 



