Unio distortus (Bean) and Alasmodon vetustus (Brown), 107 
as just above the pseudocardinal teeth, especially in young 
and middle-aged shells, there js an obvious subretusion of the 
dorsal margin, which seems to represent a formerly much 
larger and wider lunule, now reduced in size owing to the 
retrogression of the pseudocardinals and consequent approxi- 
mation of the dorsal margins of the two valves. 
Should it be ultimately proved, by the study of further 
casts, and:-more perfect examples”of the shell, that Unio 
distortus is to be referred to Margaritana, it will be of consider- 
able interest, as this species sould be the oldest known, so far, 
of this genus, one species of which being at the present time 
circumboreal, inhabiting, according to the best authorities, 
all Europe, except the southernmost portion, northern Asia, 
Japan, northern North America, and Iceland. The amended 
description, based on the specimens in the British and Manches- 
ter Museums, is given below, with synonym :— 
Margaritana 2? distorta ee 1836). [Plate IX., figs. 
rand-2.;- pl. Bz; fig. 
1836. Unio distortus Bean. Bee Niattaphiisis sca pass n05 
fig. 53. 
1843. Unio distortus Morris. ‘Cat. Brit. Foss.’, p. 105. 
843. Pachyodon vetustus, Brown. ‘Ann. Mag. N. H.’, xi, 
p.'395, pl. 16x, fig. 7 
1849. oe vestustas Brown. ‘Foss. Conch.’, p. 181, 
I ay es HTS3 3 COl 
1854. Sue etusta Morris. ‘ Cat. Brit. Foss.’, Ed. 2, p. 190 
1856. Umio -distortus. King. ‘Ann. Mag. N: H.’ (Ser. 2), 
RVING Pe 55s sple 4), die. 37. 
Shell* transversely elongated, compressed; beaks in- 
curved and situated near the anterior extremity ; umbonal 
region not prominent; lunule distinct, narrow and about 
one-fifth the length of the shell; ligament nearly half the 
length of the shell; anterior side short, rounded ; posterior 
side long, broad, and rounded at the extremity, pinched up 
above into a somewhat blunt keel or ridge; hinge and basal 
lines slightly curved ; external surface with well-defined con- 
centric lines of growth. 
Dimensions: Length, 5 inches; height, 2} inches, thick- 
ness, uncertain (Brown’s shell is r inch). 
Geol. Horizon: Upper Estuarine Series (Upper Sandstone 
and Shale of Phillips). Plentiful at the top of a rich bed of 
oolitic plants. 
Locality : Gristhorpe Bay, Yorkshire. 
(To be continued). 
* Owing to its uncompressed condition, Brown’s specimen shews the 
true form of the species better than Bean's type. 
1gtt Feb. 1. 
