C REX ELLA. 131 



and conical ; and the foot is a very extraordinary organ. 

 This is formed of two parts : one is the stalk, of a cylin- 

 drical form ; and from the upper end of it, as if from a 

 sheath, issues a tongue-shaped disk which serves for 

 crawling. The animal does not spin a thick byssus, 

 like Modiolaria, but secretes only a single slight thread 

 or filament as a point of attachment, and by means of 

 which it holds itself suspended in the water. 



Herrmannsen supposed that the genus was synony- 

 mous with Limopsis and belonged to the Arcidae. Mac- 

 gillivray placed it with Lima and Anomia in his family 

 Pectinina. 



1. Crenella rhom'bea"*, Berkeley. 



Modiola rhombea, Berk, in Zool. Journ. iii. p. 229. C. rhombea, F. & H. 

 ii. p. 208, pl.xlv. f.3. 



Shell obliquely rhoniboidal, gibbous, rather solid, somewhat 

 glossy and iridescent : sculpture, 60-70 fine longitudinal ribs, 

 crossed by 12-15 transverse plates; the former radiate from 

 the direction of the beaks, and occasionally bifurcate or 

 branch off towards the margin ; the latter form imbricated 

 ridges, and are stronger on the posterior slope : colour pearl- 

 white : epidermis extremely thin and easily rubbed off, pale 

 yellow : margins truncate or obtusely rounded on the anterior 

 side, slightly incurved in front, produced into a semicircular 

 lobe on the posterior side, and forming a wing or arched crest 

 behind: bysscd sinus small: beaks globular and prominent, 

 placed close to the anterior margin, minutely striate in a 

 transverse direction, but not sculptured in any other way : 

 ligament narrow but thick, yellowish-brown, occupying about 

 two-thirds of the hinge-line : hinge-line straight : hinge-plate 

 rather broad and strong, finely crenulated: hinge furnished with 

 a rather large wedge-shaped serrated tooth in each valve, one of 

 which interlocks within the other : inside nacreous and glossy, 

 showing distinctly the impression of the ribs ; inner margins 

 notched all round : muscular scars very slight. L. 0-13. B. 0-2. 



Habitat : Rocks and gravelly bottoms, from low- 



* Ehomboidal. 



