52 L. RICHARDSON AND J. W. TUTCHER : PTEROMYA CROWCOMBEIA 
on the absence of these features — makes no mention of the curiously 
" thickened " margin and recurved posterior portions of the valve 
(see plate VIIL, fig. 2). On the left valve the ribbing ends abruptly 
at the ridge (fig. 16), but on the right valve it is continuous to the 
" thickened " margin. The test is extremely thin. On young forms 
the ribbing is very pronounced and regular, and the presence or absence 
of the thin test does not affect these features on the cast. On the other 
hand, on older specimens the ribbing on the later-added portion of the 
shell is much less pronounced. 
PLEUROMYA TATEI nom. nOV. 
Plate VIIL, figs. :3«, h, and c. 
1871. Myacites niuscidoides, Geol. Surv., Phillips, Geohr/y of Oxford, 
p. 107, plate vii., fig. 06. 
1876. Tate and Blake, The Yorkshire Lias, p. 406, plate xiii., fig. 10. 
Remarks. — The foim to which Tate applied the specific name of 
croivcornbeia is common in and characteristic of the Pleuromya-Beds 
(the lower portion of the Os^rea-Beds auctt.) of the Lower Lias over a 
wide area, indeed from Yorkshire to the Dorset coast. 
In Moore's Pteromya crowco)nbeia the valves are dissimilar : in 
Pleiiromya tatei they are similar. The ornamentation of both valves 
of the latter species is similar to that of the left valve only of the 
former. In addition to this difference the Lower-Lias shell is very 
much larger. As in the case of the left valve of the Rhretic shell, the 
ribbing on both valves of the Lower-Lias shell terminates at the 
posterior ridge, beyond which is a narrow smooth area. 
Associated with the above-described typical form of Pleuroinya 
tatei are specimens (var. altior nov.) differing therefrom in having 
more central umbones, a stronger posteriorly reflected margin and a 
less defined ridge (plate VIIL, fig. 4). In spite of these differences we 
do not consider it desirable to separate this form specifically from the 
above. 
In the Langport Beds or White Lias proper, the Pleuroinija that 
is commonest resembles in shape Pleuromya tatei var. altior. A con- 
siderable number of specimens has been examined, but in no case has 
any indication of ribs been observed. This may be due to the condition 
