119 
ON UNIO DISTORTUS BEAN, AND ALASMODON 
VETUSTUS BROWN, FROM THE UPPER ESTUA- 
RINE BEDS OF GRISTHORPE, YORKS. 
J. WILFRID JACKSON, F.G.S., 
Manchester Museum. 
(Continued from page 107). 
With regard to Unio humatus Brown, this is very simi- 
lar to distortus Bean, at its anterior end, and has evidently 
suffered a good deal of erosion here as the back of the anterior 
adductor muscular scar is visible as in distortus. It differs, 
however, from that species in being much more inflated in the 
umbonal region, as well as in being much shorter posteriorly. 
Some allowance, however, must be made for a portion which has 
been broken off this end. This fact was evidently not noticed 
by Brown when he described and figured the species, as he 
gives the posterior side as sub-acute. 
Like distovtus, the shell possesses a narrow, but quite 
distinct lunule. The ligament, too, is fairly well preserved, 
and extends from the umbones to a point a little short of the 
postero-dorsal corner. The shell, which is somewhat crushed, 
is quite adult ; in fact, judging from the frequent interrupted 
growth-lines along the anterior and ventral margins,* it appears 
to have entered a gerontic stage. In its short antero-posterior 
diameter, coupled with the great inflation of the umbonal 
region, the species presents the features usually met with in 
sluggish water forms of the present day living well out of the 
reach of any strong currents. 
With the exception of distortus, there appears to be no other 
species described from the Yorkshire Estuarine Beds, with 
which it can be compared ; it seems, therefore, advisable to 
retain Brown’s name Unio humatus for this form until more 
specimens are met with. 
The amended description and synonymy of the species is 
as follows :— 
Unio humatus Brown, 184?. [Plate IX., fig. 3]. 
1843. Pachyodon kamatus Brown. ‘ Ann. Mag Nat. Hist.’, 
Sir, P-. 305, pl. 1Ox., fea. 
1849. Unio humatus Brown. ‘ Foss. Conch.’, p. 179, pl. 72x., 
fig. 18. 
1854. Cardinia hamata Morris. ‘Cat. Brit. Foss.’, Ed. 
p- Igo. 
Shell oblong ovate, considerably inflated, beaks large, 
slightly incurved, and situated centrally; umbonal region 
Nd 
* For the reason already stated, the posterior margin is not available 
for examination. 
rgt1 Mar. 1. 
