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. tennis, M. brevis and M. litterata, all 

 of which group he believes to be Psammobia', 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 : 



QUENSTEDTIA. Hinge in the left valve consisting of one obtuse, 

 oblong and transverse tooth, slightly compressed from above, situated 

 beneath the umbo and received into a corresponding oblong pit in 

 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 line 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 



