54 Mr. J. Lycett on the genus Quenstedtia. 



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- 

 tromi/a mactroides with M. tenuis, M. bi'evis and M. Iitte7-ata, all 

 of which group he believes to be Psammobice; he also states that 

 the group has a small cardinal tooth in each valve, which he 

 regards as an abnormal variation from the dentition oi Psammubia 

 vespej'tina, 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 oi Mactromya 7nactroides, 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 Psamniobia : 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 liinge-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 oi 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 

 aggregate of these characters, it must be admitted, fully justifies 

 the separation of Quenstedtia from Psammobia; and it yet remains 

 to be demonstrated that true Psanimobi<2 are found in any rocks 

 older than the Tertiary system, none of the so-called Jurassic 

 Psammobice having hitherto exhibited the characters of that genus 

 free from ambiguity. 



