MALACOZOA. TROPIOPODA. LAMELLIBHANCHIATA. 229 



parallel to the hinge six-twelfths, height from the umho to the 

 margin ten-twelfths. 



A single hroken valve found among shell sand, from the 

 Bay of Cruden, sent by Mr. Alexander Murray, in November, 

 1842. 



Pecten fragilis, Mont. Test. Brit. Suppl. 62. Pecten bullatus, 

 Chemnitz, Conch, vii. 342, PL 68. f. 649, 3. Lima bullata, Turt. 

 Brit. Biv. 217. PI. 17. f. 4, 5. Lima fragilis. Flem. Brit Anim. 

 388, 



GENUS 3. CRENELLA. 



Shell free, subelliptical, convex, equivalve, isomeral, 

 nacreous ; the hinge with a single abrupt fold or tooth 

 in each valve, and a sinus under the umbo ; the ligament 

 external, in a narrow groove parallel to the hinge- 

 margin ; the umbones prominent and contiguous ; the 

 valves radiatingly striate, and concentrically striulate. 



1. Crenella decussdta. Decussated Crenella* 



Shell suborbicular, more extended in the direction from the 

 umbones, being somewhat elliptical, equivalve, isomeral ; the 

 valves convex, rather thin, pellucid, nacreous, radiatingly 

 striate, and marked with concentric striulae, as well as some- 

 what regularly spaced growth-lines or ridges, producing a de- 

 cussated appearance ; the umbones prominent, a little curved 

 to one side at the point, contiguous, and central ; the hinge 

 thin, with a sinus and a thick angular fold in each valve, under 

 the umbo, between which and the margin is a narrow groove 

 for the ligament ; the inner surface glossy and nacreous, but 

 not presenting perceptible muscular impressions ; the margin 

 thinner and crenulate, especially toward the hinge. 



Of this very beautiful shell two specimens, from which the 

 above description has been taken, were found among shell 

 sand, from the Bay of Cruden, sent by Mr. Alex. Murray, in 

 November, 1 842. One, entire, but only a third of a twelfth 

 of an inch in diameter, found by Miss Marion Macgillivray, 

 is nearly quite orbicular ; the other, a single valve, found by 

 my son, Paul Howard, is a tenth of an inch in diameter, 

 and somewhat elongated, or subelliptical. 



The species was discovered in sand near Dunbar, by Captain 

 Laskey. It was accurately described by Montagu, under the 

 name of Mytilus decussatus, from a single valve. Being obvi- 

 ously not a Mytilus, Captain Brown named it Crenella elliptica, 

 U 



