136 Mr. J. Lycett on the genus Quenstedtia. 
troides; the figures representing the shell and cast of the 
interior under different aspects : upon the same plate (No. 5) are 
placed figures of the recent Psammobia vespertina for comparison ; 
the conclusion drawn by the author from this comparison is that 
Mactromya mactroides is a Psammobia. It will be observed 
that in these figures, the author has altogether omitted one of 
the most essential points of comparison necessary to establish a 
generic identity, the hinges not being exhibited; he has, however, 
given casts of the interiors of the valves in both the shells, in- 
cluding the siphonal, pallial and muscular scars, but these afford 
no information respecting the hinge. The author arranges Mac- 
tromya mactroides with M. tenuis, M. brevis and M. litterata, all 
of which group he believes to be Psammobie; he also states that 
the group has a small! cardinal tooth in each valve, which he 
regards as an abnormal variation from the dentition of Psammobia 
vespertina, which has two teeth in each valve. As the author 
does not state expressly that he has cleared and exposed the 
hinge in each valve of Mactromya mactroides, and as he is careful 
to record similar facts relating to other genera, I am led to infer 
that his knowledge of the hinge in the shell in question has 
been derived either from the partial exposure exhibited by the 
valves when in contact, or from other imperfect evidence. How- 
ever this may be, it is certain that the hinge is altogether unlike 
that of Psammobia; to avoid the trouble of reference, I subjoin 
the hinge characters of the fossil shell :— 
Quensteptia. Hinge in the left valve consisting of one obtuse, 
oblong and transverse tooth, slightly compressed from above, situated 
beneath the unbo and received into a corresponding oblong pitin 
the hinge-plate of the opposite valve. 
There is therefore no tooth in the right valve and consequently 
no pit in the left valve. This kind of hinge, which so nearly 
resembles that of the fossil genus Myoconcha, is altogether 
distinct from that of Psammobia, with its two grooved, diverging 
hinge-teeth in each valve. 
The ligament is received into a narrow, lengthened and deep 
area posterior to the umbones ;—the shell is therefore destitute of 
the elevated nymphal plate of Psammobia. 
The siphonal flexure, as may be observed in the figure given 
by M. Terquem, is less considerable than in Psammobia, and, 
unlike that genus, it is united posteriorly to the pallial lme only 
at its extremity, so that with the pallial line it forms a narrow 
tongue, the upper and lower borders of which are limited by the 
gradual convergence of the two lines : in Psammobia the siphonal 
and pallial lines are united in a position nearly vertical beneath 
the umbo, and proceed posteriorly united into a single line. The 
