THE NAUTILUS. 109 



on pi, and Musculia have the same, still less marked, and in 

 nearly all species the laminae are very slight. 



In Pisidium, having the anterior part of the mussel larger, 

 with a few exceptions, the anterior laminae, al and all, are 

 generally somewhat longer and stouter than pi and pll. 

 The whole hinge and the laminae, with their cusps, show great 

 differences of configuration, that it is out of place here even 

 to sketch the principal forms; pi. 3, figs. 4-6 show a few of 

 them, and some notes will be found later on. The outer 

 laminae of the right valve, alii and pill, are generally much 

 smaller than al and pi, in some species constant, in others 

 occasionally only vestigial or wanting. 



Some laminae, especially the stouter ones, and again especi- 

 ally those of the right valve, on the surfaces surrounding the 

 grooves between them, are microscopically rugulose. It may 

 be noted in this connection that the feature is especially de- 

 veloped, locally, with species of Spharium, e. g. solidulum, sul- 

 catum, rivicola, etc. In these, on the upper face of al, there 

 is a circumscribed, rather small area, somewhat concave, and 

 often walled in by a more or less raised rim, not only rugu- 

 lose but densely beset with separate, round, wart- or tubercle- 

 like prominences; the opposite, lower face of alii shows the 

 same, though less marked : the place where the tip of the 

 cusp of all enters; yet the latter is smooth or shows only 

 very slight rugosity. 



Reversed Hinges. - - In some specimens the hinges are re- 

 versed, that is : the teeth of the left valve have the formation 

 of the right ones, and vice versa, as the normal ones would 

 be seen in a mirror reflection. The hinge is either (1) totally 

 reversed, each valve showing all the features of the opposite 

 one, or (2) only the anterior part is reversed, namely the 

 cardinals and the anterior laminfe, or (3) only the posterior 

 laminae. No specimen has been seen in which only the an- 

 terior laminae were reversed, or only the cardinals, or the 

 posterior laminae plus the cardinals. This is certainly of in- 

 terest morphologically. Such hinges are quite frequent with 

 the species of Sphccrium, e. g. striatinum, solidulum, etc. ; 

 twenty-five or even more per cent of the specimens in a lot 



