142 GASTROCIIiENID.'E. 



this species, owiug to the erroneous but current beHcf that 

 arctica is distiuguished from rugosa by being armed with 

 spinous scales. This character, however, is shared, although 

 less strongly so, by the young of the latter, and is com- 

 jjletely lost in the aged examples of either species. Neither 

 Avill the presence of teeth upon the hinge margin of arctica 

 suffice for its ready separation, even though conjoined 

 with the previously mentioned character, since the young 

 rugosa have likewise teeth, although most feebly deve- 

 loped. It is requisite, then, to discover some permanent 

 distinctive characteristic, which may assist the eye in the 

 determination of the two species, that organ frequently 

 enabling the collecting naturalist to ascertain the aggregate 

 value of specific differences, Avhich neither his tongue nor 

 his pen can analytically define. This method, however 

 allowable to collectors, is most reprehensible in authors, too 

 many of whom have indolently preferred trusting the esta- 

 blishment of their discoveries to the skilful pencil of the 

 artist, rather than themselves laboriously pourtray with 

 their pen those several features, from the combination of 

 which specific identity is constituted ; thus entailing upon 

 every author of a cyclopaedia of species the necessity of 

 personally examining each individual one, a labour which, 

 however possible and desirable in a local or partial Fauna, 

 must, in a general descriptive catalogue, be practically 

 unattainable. 



The tangible mark of distinction between arctica and 

 rugosa consists in the constant presence of an excavated 

 lunule in front of the beaks, which are moreover acute, 

 leaning forward, and, when viewed in front, sufficiently 

 prominent. The anterior extremity is more or less cunei- 

 form, and is always attenuated; whereas in rugosa that 

 portion is usually rounded and frequently broad : in that 



