PATELLA. 



TESTA pier umque ovata, depresso vel subde- 

 presso-conica, subtus concava, vertice semper 

 antico, plerumque subcentrali, raritis admodum 

 marginal i vel submarginali : impressio muscu- 

 laris elliptica, antiee interupta. 



VateIjLa^ when detached from all the other genera 

 which were associated with it under that name by Linnean 

 authors, forms still a Genus very comprehensive in num- 

 bers, and though well characterized as a Genus, the study 

 of it is rendered extremely difficult, on account of the 

 variations to which the species are liable from peculiarity 

 of position or situation. This observation is suggested by 

 the fact, that the P. caerulea is extremely regular and 

 thin when it has lived upon the leaves and stems of sea- 

 weed 5 and, on the contrary, irregular when attached to 

 the roots : we have also reason to believe that a like 

 difference of situation is the cause of the great difference 

 in character between P. compressa and P. miniata; but we 

 are confident that they ought to be considered as varieties 

 of the same, for we possess specimens in which, from the 

 vertex to about half an inch from the margin, the charac- 

 ters are those of compressa, while the remainder of the same 

 identical specimen is indisputably a well characterized 

 P. miniata : it is remarkable that Lamarck should have 

 observed the same fact, but considers it in some measure 

 inexplicable. 



Shell generally ovate, sometimes oblong ; more or less 

 depressedly conical, rarely of a pyramidal form; concave 

 beneath in proportion as the vertex is convex. The apex 

 or vertex is sometimes very nearly central, always more 

 or less anterior, very seldom marginal, or indeed nearly so; 

 when curved, generally towards the head of the animal: 

 a character by which Patella may be distinguished from 

 all the other shells that have been associated with it. 



